[ Note: Clause [expr] defines the syntax, order of evaluation, and meaning of expressions.62 An expression is a sequence of operators and operands that specifies a computation. An expression can result in a value and can cause side effects. — end note ]
[ Note: Operators can be overloaded, that is, given meaning when applied to expressions of class type or enumeration type. Uses of overloaded operators are transformed into function calls as described in [over.oper]. Overloaded operators obey the rules for syntax and evaluation order specified in Clause [expr], but the requirements of operand type and value category are replaced by the rules for function call. Relations between operators, such as ++a meaning a+=1, are not guaranteed for overloaded operators. — end note ]
Clause [expr] defines the effects of operators when applied to types for which they have not been overloaded. Operator overloading shall not modify the rules for the built-in operators, that is, for operators applied to types for which they are defined by this Standard. However, these built-in operators participate in overload resolution, and as part of that process user-defined conversions will be considered where necessary to convert the operands to types appropriate for the built-in operator. If a built-in operator is selected, such conversions will be applied to the operands before the operation is considered further according to the rules in Clause [expr]; see [over.match.oper], [over.built].
If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined. [ Note: Treatment of division by zero, forming a remainder using a zero divisor, and all floating-point exceptions vary among machines, and is sometimes adjustable by a library function. — end note ]
If an expression initially has the type “reference to T” ([dcl.ref], [dcl.init.ref]), the type is adjusted to T prior to any further analysis. The expression designates the object or function denoted by the reference, and the expression is an lvalue or an xvalue, depending on the expression. [ Note: Before the lifetime of the reference has started or after it has ended, the behavior is undefined (see [basic.life]). — end note ]
If a prvalue initially has the type “cv T”, where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.
[ Note: An expression is an xvalue if it is:
the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to object type,
a cast to an rvalue reference to object type,
a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue, or
a .* pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member.
In general, the effect of this rule is that named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not. — end note ]
[ Example:
struct A {
int m;
};
A&& operator+(A, A);
A&& f();
A a;
A&& ar = static_cast<A&&>(a);
The expressions f(), f().m, static_cast<A&&>(a), and a + a are xvalues. The expression ar is an lvalue. — end example ]
Whenever a glvalue expression appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue, array-to-pointer, or function-to-pointer standard conversions are applied to convert the expression to a prvalue. [ Note: Because cv-qualifiers are removed from the type of an expression of non-class type when the expression is converted to a prvalue, an lvalue expression of type const int can, for example, be used where a prvalue expression of type int is required. — end note ]
Whenever a prvalue expression appears as an operand of an operator that expects a glvalue for that operand, the temporary materialization conversion is applied to convert the expression to an xvalue.
Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way. The purpose is to yield a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions, which are defined as follows:
If either operand is of scoped enumeration type, no conversions are performed; if the other operand does not have the same type, the expression is ill-formed.
If either operand is of type long double, the other shall be converted to long double.
Otherwise, if either operand is double, the other shall be converted to double.
Otherwise, if either operand is float, the other shall be converted to float.
Otherwise, the integral promotions shall be performed on both operands.63 Then the following rules shall be applied to the promoted operands:
If both operands have the same type, no further conversion is needed.
Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank shall be converted to the type of the operand with greater rank.
Otherwise, if the operand that has unsigned integer type has rank greater than or equal to the rank of the type of the other operand, the operand with signed integer type shall be converted to the type of the operand with unsigned integer type.
Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, the operand with unsigned integer type shall be converted to the type of the operand with signed integer type.
Otherwise, both operands shall be converted to the unsigned integer type corresponding to the type of the operand with signed integer type.
In some contexts, an expression only appears for its side effects. Such an expression is called a discarded-value expression. The array-to-pointer and function-to-pointer standard conversions are not applied. The lvalue-to-rvalue conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:
( expression ), where expression is one of these expressions,
conditional expression where both the second and the third operands are one of these expressions, or
comma expression where the right operand is one of these expressions.
[ Note: Using an overloaded operator causes a function call; the above covers only operators with built-in meaning. — end note ] If the expression is a prvalue after this optional conversion, the temporary materialization conversion is applied. [ Note: If the expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary that is the result object of the lvalue-to-rvalue conversion. — end note ] The glvalue expression is evaluated and its value is discarded.
The values of the floating operands and the results of floating expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby.64
The cv-combined type of two types T1 and T2 is a type T3 similar to T1 whose cv-qualification signature is:
for every i>0, cv3i is the union of cv1i and cv2i;
if the resulting cv3i is different from cv1i or cv2i, then const is added to every cv3k for 0<k<i.
[ Note: Given similar types T1 and T2, this construction ensures that both can be converted to T3. — end note ]
The composite pointer type of two operands p1 and p2 having types T1 and T2, respectively, where at least one is a pointer or pointer to member type or std::nullptr_t, is:
if both p1 and p2 are null pointer constants, std::nullptr_t;
if either p1 or p2 is a null pointer constant, T2 or T1, respectively;
if T1 or T2 is “pointer to cv1 void” and the other type is “pointer to cv2 T”, where T is an object type or void, “pointer to cv12 void”, where cv12 is the union of cv1 and cv2;
if T1 or T2 is “pointer to noexcept function” and the other type is “pointer to function”, where the function types are otherwise the same, “pointer to function”;
if T1 is “pointer to cv1 C1” and T2 is “pointer to cv2 C2”, where C1 is reference-related to C2 or C2 is reference-related to C1, the cv-combined type of T1 and T2 or the cv-combined type of T2 and T1, respectively;
if T1 is “pointer to member of C1 of type cv1 U1” and T2 is “pointer to member of C2 of type cv2 U2” where C1 is reference-related to C2 or C2 is reference-related to C1, the cv-combined type of T2 and T1 or the cv-combined type of T1 and T2, respectively;
if T1 and T2 are similar types, the cv-combined type of T1 and T2;
otherwise, a program that necessitates the determination of a composite pointer type is ill-formed.
[ Example:
typedef void *p; typedef const int *q; typedef int **pi; typedef const int **pci;
The composite pointer type of p and q is “pointer to const void”; the composite pointer type of pi and pci is “pointer to const pointer to const int”. — end example ]
8.1 Primary expressions [expr.prim]primary-expression: literal this ( expression ) id-expression lambda-expression fold-expression8.1.2 This [expr.prim.this]
The keyword this names a pointer to the object for which a non-static member function is invoked or a non-static data member's initializer ([class.mem]) is evaluated.
If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” between the optional cv-qualifier-seq and the end of the function-definition, member-declarator, or declarator. It shall not appear before the optional cv-qualifier-seq and it shall not appear within the declaration of a static member function (although its type and value category are defined within a static member function as they are within a non-static member function). [ Note: This is because declaration matching does not occur until the complete declarator is known. — end note ] Unlike the object expression in other contexts, *this is not required to be of complete type for purposes of class member access outside the member function body. [ Note: Only class members declared prior to the declaration are visible. — end note ] [ Example:
struct A {
char g();
template<class T> auto f(T t) -> decltype(t + g())
{ return t + g(); }
};
template auto A::f(int t) -> decltype(t + g());
— end example ]
The expression this shall not appear in any other context. [ Example:
class Outer {
int a[sizeof(*this)]; unsigned int sz = sizeof(*this);
void f() {
int b[sizeof(*this)];
struct Inner {
int c[sizeof(*this)]; };
}
};
— end example ]
8.1.3 Parentheses [expr.prim.paren]A parenthesized expression (E) is a primary expression whose type, value, and value category are identical to those of E. The parenthesized expression can be used in exactly the same contexts as those where E can be used, and with the same meaning, except as otherwise indicated.
8.1.5 Lambda expressions [expr.prim.lambda]lambda-expression: lambda-introducer lambda-declaratoropt compound-statement
lambda-introducer: [ lambda-captureopt ]
lambda-declarator: ( parameter-declaration-clause ) decl-specifier-seqopt noexcept-specifieropt attribute-specifier-seqopt trailing-return-typeopt
Lambda expressions provide a concise way to create simple function objects. [ Example:
#include <algorithm>
#include <cmath>
void abssort(float* x, unsigned N) {
std::sort(x, x + N, [](float a, float b) { return std::abs(a) < std::abs(b); });
}
— end example ]
8.1.5.1 Closure types [expr.prim.lambda.closure]The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.
The closure type is declared in the smallest block scope, class scope, or namespace scope that contains the corresponding lambda-expression. [ Note: This determines the set of namespaces and classes associated with the closure type ([basic.lookup.argdep]). The parameter types of a lambda-declarator do not affect these associated namespaces and classes. — end note ] The closure type is not an aggregate type ([dcl.init.aggr]). An implementation may define the closure type differently from what is described below provided this does not alter the observable behavior of the program other than by changing:
the size and/or alignment of the closure type,
whether the closure type is trivially copyable,
whether the closure type is a standard-layout class, or
whether the closure type is a POD class.
An implementation shall not add members of rvalue reference type to the closure type.
[ Example:
auto monoid = [](auto v) { return [=] { return v; }; };
auto add = [](auto m1) constexpr {
auto ret = m1();
return [=](auto m2) mutable {
auto m1val = m1();
auto plus = [=](auto m2val) mutable constexpr
{ return m1val += m2val; };
ret = plus(m2());
return monoid(ret);
};
};
constexpr auto zero = monoid(0);
constexpr auto one = monoid(1);
static_assert(add(one)(zero)() == one());
auto two = monoid(2);
assert(two() == 2); static_assert(add(one)(one)() == two()); static_assert(add(one)(one)() == monoid(2)());
— end example ]
The closure type for a non-generic lambda-expression with no lambda-capture has a conversion function to pointer to function with C++ language linkage having the same parameter and return types as the closure type's function call operator. The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification. The value returned by this conversion function is the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator. F is a constexpr function if the function call operator is a constexpr function. For a generic lambda with no lambda-capture, the closure type has a conversion function template to pointer to function. The conversion function template has the same invented template-parameter-list, and the pointer to function has the same parameter types, as the function call operator template. The return type of the pointer to function shall behave as if it were a decltype-specifier denoting the return type of the corresponding function call operator template specialization.
[ Note: If the generic lambda has no trailing-return-type or the trailing-return-type contains a placeholder type, return type deduction of the corresponding function call operator template specialization has to be done. The corresponding specialization is that instantiation of the function call operator template with the same template arguments as those deduced for the conversion function template. Consider the following:
auto glambda = [](auto a) { return a; };
int (*fp)(int) = glambda;
The behavior of the conversion function of glambda above is like that of the following conversion function:
struct Closure {
template<class T> auto operator()(T t) const { ... }
template<class T> static auto lambda_call_operator_invoker(T a) {
...
}
template<class T> using fptr_t =
decltype(lambda_call_operator_invoker(declval<T>())) (*)(T);
template<class T> operator fptr_t<T>() const
{ return &lambda_call_operator_invoker; }
};
— end note ]
[ Example:
void f1(int (*)(int)) { }
void f2(char (*)(int)) { }
void g(int (*)(int)) { } void g(char (*)(char)) { }
void h(int (*)(int)) { } void h(char (*)(int)) { }
auto glambda = [](auto a) { return a; };
f1(glambda); f2(glambda); g(glambda); h(glambda); int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; };
— end example ]
The value returned by any given specialization of this conversion function template is the address of a function F that, when invoked, has the same effect as invoking the generic lambda's corresponding function call operator template specialization. F is a constexpr function if the corresponding specialization is a constexpr function. [ Note: This will result in the implicit instantiation of the generic lambda's body. The instantiated generic lambda's return type and parameter types shall match the return type and parameter types of the pointer to function. — end note ] [ Example:
auto GL = [](auto a) { std::cout << a; return a; };
int (*GL_int)(int) = GL; GL_int(3);
— end example ]
The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification. [ Example:
auto Fwd = [](int (*fp)(int), auto a) { return fp(a); };
auto C = [](auto a) { return a; };
static_assert(Fwd(C,3) == 3);
auto NC = [](auto a) { static int s; return a; };
static_assert(Fwd(NC,3) == 3);
— end example ]
The closure type associated with a lambda-expression has no default constructor and a deleted copy assignment operator. It has a defaulted copy constructor and a defaulted move constructor ([class.copy]). [ Note: These special member functions are implicitly defined as usual, and might therefore be defined as deleted. — end note ]
8.1.5.2 Captures [expr.prim.lambda.capture]lambda-capture: capture-default capture-list capture-default , capture-list
capture-default: & =
capture-list: capture ...opt capture-list , capture ...opt
capture: simple-capture init-capture
simple-capture: identifier & identifier this * this
init-capture: identifier initializer & identifier initializer
The body of a lambda-expression may refer to variables with automatic storage duration and the *this object (if any) of enclosing block scopes by capturing those entities, as described below.
If a lambda-capture includes a capture-default that is &, no identifier in a simple-capture of that lambda-capture shall be preceded by &. If a lambda-capture includes a capture-default that is =, each simple-capture of that lambda-capture shall be of the form “& identifier” or “* this”. [ Note: The form [&,this] is redundant but accepted for compatibility with ISO C++ 2014. — end note ] Ignoring appearances in initializers of init-captures, an identifier or this shall not appear more than once in a lambda-capture. [ Example:
struct S2 { void f(int i); };
void S2::f(int i) {
[&, i]{ }; [&, &i]{ }; [=, *this]{ }; [=, this]{ }; [i, i]{ }; [this, *this]{ }; }
— end example ]
An init-capture behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;” whose declarative region is the lambda-expression's compound-statement, except that:
if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
if the capture is by reference, the variable's lifetime ends when the closure object's lifetime ends.
[ Note: This enables an init-capture like “x = std::move(x)”; the second “x” must bind to a declaration in the surrounding context. — end note ] [ Example:
int x = 4;
auto y = [&r = x, x = x+1]()->int {
r += 2;
return x+2;
}();
auto z = [a = 42](int a) { return 1; }
— end example ]
A lambda-expression with an associated capture-default that does not explicitly capture *this or a variable with automatic storage duration (this excludes any id-expression that has been found to refer to an init-capture's associated non-static data member), is said to implicitly capture the entity (i.e., *this or a variable) if the compound-statement:
odr-uses the entity (in the case of a variable),
odr-uses this (in the case of the object designated by *this), or
names the entity in a potentially evaluated expression where the enclosing full-expression depends on a generic lambda parameter declared within the reaching scope of the lambda-expression.
[ Example:
void f(int, const int (&)[2] = {}) { } void f(const int&, const int (&)[1]) { } void test() {
const int x = 17;
auto g = [](auto a) {
f(x); };
auto g2 = [=](auto a) {
int selector[sizeof(a) == 1 ? 1 : 2]{};
f(x, selector); };
}
— end example ] All such implicitly captured entities shall be declared within the reaching scope of the lambda expression. [ Note: The implicit capture of an entity by a nested lambda-expression can cause its implicit capture by the containing lambda-expression (see below). Implicit odr-uses of this can result in implicit capture. — end note ]
An entity is captured if it is captured explicitly or implicitly. An entity captured by a lambda-expression is odr-used in the scope containing the lambda-expression. If *this is captured by a local lambda expression, its nearest enclosing function shall be a non-static member function. If a lambda-expression or an instantiation of the function call operator template of a generic lambda odr-uses this or a variable with automatic storage duration from its reaching scope, that entity shall be captured by the lambda-expression. If a lambda-expression captures an entity and that entity is not defined or captured in the immediately enclosing lambda expression or function, the program is ill-formed. [ Example:
void f1(int i) {
int const N = 20;
auto m1 = [=]{
int const M = 30;
auto m2 = [i]{
int x[N][M]; x[0][0] = i; };
};
struct s1 {
int f;
void work(int n) {
int m = n*n;
int j = 40;
auto m3 = [this,m] {
auto m4 = [&,j] { int x = n; x += m; x += i; x += f; };
};
}
};
}
struct s2 {
double ohseven = .007;
auto f() {
return [this] {
return [*this] {
return ohseven; }
}();
}
auto g() {
return [] {
return [*this] { }; }();
}
};
— end example ]
A lambda-expression appearing in a default argument shall not implicitly or explicitly capture any entity. [ Example:
void f2() {
int i = 1;
void g1(int = ([i]{ return i; })()); void g2(int = ([i]{ return 0; })()); void g3(int = ([=]{ return i; })()); void g4(int = ([=]{ return 0; })()); void g5(int = ([]{ return sizeof i; })()); }
— end example ]
An entity is captured by copy if
it is implicitly captured, the capture-default is =, and the captured entity is not *this, or
it is explicitly captured with a capture that is not of the form this, & identifier, or & identifier initializer.
For each entity captured by copy, an unnamed non-static data member is declared in the closure type. The declaration order of these members is unspecified. The type of such a data member is the referenced type if the entity is a reference to an object, an lvalue reference to the referenced function type if the entity is a reference to a function, or the type of the corresponding captured entity otherwise. A member of an anonymous union shall not be captured by copy.
Every id-expression within the compound-statement of a lambda-expression that is an odr-use of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type. [ Note: An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type. Furthermore, such an id-expression does not cause the implicit capture of the entity. — end note ] If *this is captured by copy, each odr-use of this is transformed into a pointer to the corresponding unnamed data member of the closure type, cast to the type of this. [ Note: The cast ensures that the transformed expression is a prvalue. — end note ] An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference. [ Note: The validity of such captures is determined by the lifetime of the object to which the reference refers, not by the lifetime of the reference itself. — end note ] [ Example:
void f(const int*);
void g() {
const int N = 10;
[=] {
int arr[N]; f(&N); };
}
auto h(int &r) {
return [&] {
++r; };
}
— end example ]
An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy. It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference. If declared, such non-static data members shall be of literal type. [ Example:
static_assert([](int n) { return [&n] { return ++n; }(); }(3) == 4);
— end example ] A bit-field or a member of an anonymous union shall not be captured by reference.
If a lambda-expression m2 captures an entity and that entity is captured by an immediately enclosing lambda-expression m1, then m2's capture is transformed as follows:
if m1 captures the entity by copy, m2 captures the corresponding non-static data member of m1's closure type;
if m1 captures the entity by reference, m2 captures the same entity captured by m1.
[ Example: The nested lambda expressions and invocations below will output 123234.
int a = 1, b = 1, c = 1;
auto m1 = [a, &b, &c]() mutable {
auto m2 = [a, b, &c]() mutable {
std::cout << a << b << c;
a = 4; b = 4; c = 4;
};
a = 3; b = 3; c = 3;
m2();
};
a = 2; b = 2; c = 2;
m1();
std::cout << a << b << c;
— end example ]
Every occurrence of decltype((x)) where x is a possibly parenthesized id-expression that names an entity of automatic storage duration is treated as if x were transformed into an access to a corresponding data member of the closure type that would have been declared if x were an odr-use of the denoted entity. [ Example:
void f3() {
float x, &r = x;
[=] { decltype(x) y1; decltype((x)) y2 = y1; decltype(r) r1 = y1; decltype((r)) r2 = y2; };
}
— end example ]
When the lambda-expression is evaluated, the entities that are captured by copy are used to direct-initialize each corresponding non-static data member of the resulting closure object, and the non-static data members corresponding to the init-captures are initialized as indicated by the corresponding initializer (which may be copy- or direct-initialization). (For array members, the array elements are direct-initialized in increasing subscript order.) These initializations are performed in the (unspecified) order in which the non-static data members are declared. [ Note: This ensures that the destructions will occur in the reverse order of the constructions. — end note ]
[ Note: If a non-reference entity is implicitly or explicitly captured by reference, invoking the function call operator of the corresponding lambda-expression after the lifetime of the entity has ended is likely to result in undefined behavior. — end note ]
A simple-capture followed by an ellipsis is a pack expansion. An init-capture followed by an ellipsis is ill-formed. [ Example:
template<class... Args>
void f(Args... args) {
auto lm = [&, args...] { return g(args...); };
lm();
}
— end example ]
8.1.6 Fold expressions [expr.prim.fold]An expression of the form (e1 op1 ... op2 e2) where op1 and op2 are fold-operators is called a binary fold. In a binary fold, op1 and op2 shall be the same fold-operator, and either e1 shall contain an unexpanded parameter pack or e2 shall contain an unexpanded parameter pack, but not both. If e2 contains an unexpanded parameter pack, the expression is called a binary left fold. If e1 contains an unexpanded parameter pack, the expression is called a binary right fold. [ Example:
template<typename ...Args>
bool f(Args ...args) {
return (true && ... && args); }
template<typename ...Args>
bool f(Args ...args) {
return (args + ... + args); }
— end example ]
8.2 Postfix expressions [expr.post][ Note: The > token following the type-id in a dynamic_cast, static_cast, reinterpret_cast, or const_cast may be the product of replacing a >> token by two consecutive > tokens ([temp.names]). — end note ]
8.2.1 Subscripting [expr.sub]A postfix expression followed by an expression in square brackets is a postfix expression. One of the expressions shall be a glvalue of type “array of T” or a prvalue of type “pointer to T” and the other shall be a prvalue of unscoped enumeration or integral type. The result is of type “T”. The type “T” shall be a completely-defined object type.66 The expression E1[E2] is identical (by definition) to *((E1)+(E2)) [ Note: see [expr.unary] and [expr.add] for details of * and + and [dcl.array] for details of arrays. — end note ] , except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise. The expression E1 is sequenced before the expression E2.
8.2.2 Function call [expr.call]A function call is a postfix expression followed by parentheses containing a possibly empty, comma-separated list of initializer-clauses which constitute the arguments to the function. The postfix expression shall have function type or function pointer type. For a call to a non-member function or to a static member function, the postfix expression shall be either an lvalue that refers to a function (in which case the function-to-pointer standard conversion is suppressed on the postfix expression), or it shall have function pointer type. Calling a function through an expression whose function type is different from the function type of the called function's definition results in undefined behavior ([dcl.link]). For a call to a non-static member function, the postfix expression shall be an implicit ([class.mfct.non-static], [class.static]) or explicit class member access whose id-expression is a function member name, or a pointer-to-member expression selecting a function member; the call is as a member of the class object referred to by the object expression. In the case of an implicit class member access, the implied object is the one pointed to by this. [ Note: A member function call of the form f() is interpreted as (*this).f() (see [class.mfct.non-static]). — end note ] If a function or member function name is used, the name can be overloaded, in which case the appropriate function shall be selected according to the rules in [over.match]. If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called. Otherwise, its final overrider in the dynamic type of the object expression is called; such a call is referred to as a virtual function call. [ Note: The dynamic type is the type of the object referred to by the current value of the object expression. [class.cdtor] describes the behavior of virtual function calls when the object expression refers to an object under construction or destruction. — end note ]
[ Note: If a function or member function name is used, and name lookup does not find a declaration of that name, the program is ill-formed. No function is implicitly declared by such a call. — end note ]
If the postfix-expression designates a destructor, the type of the function call expression is void; otherwise, the type of the function call expression is the return type of the statically chosen function (i.e., ignoring the virtual keyword), even if the type of the function actually called is different. This return type shall be an object type, a reference type or cv void.
When a function is called, each parameter ([dcl.fct]) shall be initialized ([dcl.init], [class.copy], [class.ctor]) with its corresponding argument. If the function is a non-static member function, the this parameter of the function shall be initialized with a pointer to the object of the call, converted as if by an explicit type conversion. [ Note: There is no access or ambiguity checking on this conversion; the access checking and disambiguation are done as part of the (possibly implicit) class member access operator. See [class.member.lookup], [class.access.base], and [expr.ref]. — end note ] When a function is called, the parameters that have object type shall have completely-defined object type. [ Note: this still allows a parameter to be a pointer or reference to an incomplete class type. However, it prevents a passed-by-value parameter to have an incomplete class type. — end note ] It is implementation-defined whether the lifetime of a parameter ends when the function in which it is defined returns or at the end of the enclosing full-expression. The initialization and destruction of each parameter occurs within the context of the calling function. [ Example: The access of the constructor, conversion functions or destructor is checked at the point of call in the calling function. If a constructor or destructor for a function parameter throws an exception, the search for a handler starts in the scope of the calling function; in particular, if the function called has a function-try-block with a handler that could handle the exception, this handler is not considered. — end example ]
The postfix-expression is sequenced before each expression in the expression-list and any default argument. The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter. [ Note: All side effects of argument evaluations are sequenced before the function is entered (see [intro.execution]). — end note ] [ Example:
void f() {
std::string s = "but I have heard it works even if you don't believe in it";
s.replace(0, 4, "").replace(s.find("even"), 4, "only").replace(s.find(" don't"), 6, "");
assert(s == "I have heard it works only if you believe in it"); }
— end example ] [ Note: If an operator function is invoked using operator notation, argument evaluation is sequenced as specified for the built-in operator; see [over.match.oper]. — end note ] [ Example:
struct S {
S(int);
};
int operator<<(S, int);
int i, j;
int x = S(i=1) << (i=2);
int y = operator<<(S(j=1), j=2);
After performing the initializations, the value of i is 2 (see [expr.shift]), but it is unspecified whether the value of j is 1 or 2. — end example ]
The result of a function call is the result of the operand of the evaluated return statement in the called function (if any), except in a virtual function call if the return type of the final overrider is different from the return type of the statically chosen function, the value returned from the final overrider is converted to the return type of the statically chosen function.
[ Note: A function can change the values of its non-const parameters, but these changes cannot affect the values of the arguments except where a parameter is of a reference type ([dcl.ref]); if the reference is to a const-qualified type, const_cast is required to be used to cast away the constness in order to modify the argument's value. Where a parameter is of const reference type a temporary object is introduced if needed ([dcl.type], [lex.literal], [lex.string], [dcl.array], [class.temporary]). In addition, it is possible to modify the values of non-constant objects through pointer parameters. — end note ]
A function can be declared to accept fewer arguments (by declaring default arguments) or more arguments (by using the ellipsis, ..., or a function parameter pack ([dcl.fct])) than the number of parameters in the function definition. [ Note: This implies that, except where the ellipsis (...) or a function parameter pack is used, a parameter is available for each argument. — end note ]
When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg. [ Note: This paragraph does not apply to arguments passed to a function parameter pack. Function parameter packs are expanded during template instantiation ([temp.variadic]), thus each such argument has a corresponding parameter when a function template specialization is actually called. — end note ] The lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the argument expression. An argument that has type cv std::nullptr_t is converted to type void*. After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer to member, or class type, the program is ill-formed. Passing a potentially-evaluated argument of class type having a non-trivial copy constructor, a non-trivial move constructor, or a non-trivial destructor, with no corresponding parameter, is conditionally-supported with implementation-defined semantics. If the argument has integral or enumeration type that is subject to the integral promotions, or a floating-point type that is subject to the floating-point promotion, the value of the argument is converted to the promoted type before the call. These promotions are referred to as the default argument promotions.
A function call is an lvalue if the result type is an lvalue reference type or an rvalue reference to function type, an xvalue if the result type is an rvalue reference to object type, and a prvalue otherwise.
8.2.3 Explicit type conversion (functional notation) [expr.type.conv]If the initializer is a parenthesized single expression, the type conversion expression is equivalent (in definedness, and if defined in meaning) to the corresponding cast expression. If the type is cv void and the initializer is (), the expression is a prvalue of the specified type that performs no initialization. Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized with the initializer. For an expression of the form T(), T shall not be an array type.
8.2.4 Pseudo destructor call [expr.pseudo]The use of a pseudo-destructor-name after a dot . or arrow -> operator represents the destructor for the non-class type denoted by type-name or decltype-specifier. The result shall only be used as the operand for the function call operator (), and the result of such a call has type void. The only effect is the evaluation of the postfix-expression before the dot or arrow.
The left-hand side of the dot operator shall be of scalar type. The left-hand side of the arrow operator shall be of pointer to scalar type. This scalar type is the object type. The cv-unqualified versions of the object type and of the type designated by the pseudo-destructor-name shall be the same type. Furthermore, the two type-names in a pseudo-destructor-name of the form
nested-name-specifieropt type-name :: ~ type-name
shall designate the same scalar type (ignoring cv-qualification).
8.2.5 Class member access [expr.ref]A postfix expression followed by a dot . or an arrow ->, optionally followed by the keyword template ([temp.names]), and then followed by an id-expression, is a postfix expression. The postfix expression before the dot or arrow is evaluated;67 the result of that evaluation, together with the id-expression, determines the result of the entire postfix expression.
For the first option (dot) the first expression shall be a glvalue having complete class type. For the second option (arrow) the first expression shall be a prvalue having pointer to complete class type. The expression E1->E2 is converted to the equivalent form (*(E1)).E2; the remainder of [expr.ref] will address only the first option (dot).68 In either case, the id-expression shall name a member of the class or of one of its base classes. [ Note: Because the name of a class is inserted in its class scope (Clause [class]), the name of a class is also considered a nested member of that class. — end note ] [ Note: [basic.lookup.classref] describes how names are looked up after the . and -> operators. — end note ]
Abbreviating postfix-expression.id-expression as E1.E2, E1 is called the object expression. If E2 is a bit-field, E1.E2 is a bit-field. The type and value category of E1.E2 are determined as follows. In the remainder of [expr.ref], cq represents either const or the absence of const and vq represents either volatile or the absence of volatile. cv represents an arbitrary set of cv-qualifiers, as defined in [basic.type.qualifier].
If E2 is declared to have type “reference to T”, then E1.E2 is an lvalue; the type of E1.E2 is T. Otherwise, one of the following rules applies.
If E2 is a static data member and the type of E2 is T, then E1.E2 is an lvalue; the expression designates the named member of the class. The type of E1.E2 is T.
If E2 is a non-static data member and the type of E1 is “cq1 vq1 X”, and the type of E2 is “cq2 vq2 T”, the expression designates the named member of the object designated by the first expression. If E1 is an lvalue, then E1.E2 is an lvalue; otherwise E1.E2 is an xvalue. Let the notation vq12 stand for the “union” of vq1 and vq2; that is, if vq1 or vq2 is volatile, then vq12 is volatile. Similarly, let the notation cq12 stand for the “union” of cq1 and cq2; that is, if cq1 or cq2 is const, then cq12 is const. If E2 is declared to be a mutable member, then the type of E1.E2 is “vq12 T”. If E2 is not declared to be a mutable member, then the type of E1.E2 is “cq12 vq12 T”.
If E2 is a (possibly overloaded) member function, function overload resolution is used to determine whether E1.E2 refers to a static or a non-static member function.
If it refers to a static member function and the type of E2 is “function of parameter-type-list returning T”, then E1.E2 is an lvalue; the expression designates the static member function. The type of E1.E2 is the same type as that of E2, namely “function of parameter-type-list returning T”.
Otherwise, if E1.E2 refers to a non-static member function and the type of E2 is “function of parameter-type-list cv ref-qualifieropt returning T”, then E1.E2 is a prvalue. The expression designates a non-static member function. The expression can be used only as the left-hand operand of a member function call ([class.mfct]). [ Note: Any redundant set of parentheses surrounding the expression is ignored ([expr.prim]). — end note ] The type of E1.E2 is “function of parameter-type-list cv returning T”.
If E2 is a nested type, the expression E1.E2 is ill-formed.
If E2 is a member enumerator and the type of E2 is T, the expression E1.E2 is a prvalue. The type of E1.E2 is T.
If E2 is a non-static data member or a non-static member function, the program is ill-formed if the class of which E2 is directly a member is an ambiguous base ([class.member.lookup]) of the naming class ([class.access.base]) of E2. [ Note: The program is also ill-formed if the naming class is an ambiguous base of the class type of the object expression; see [class.access.base]. — end note ]
8.2.6 Increment and decrement [expr.post.incr]The value of a postfix ++ expression is the value of its operand. [ Note: The value obtained is a copy of the original value — end note ] The operand shall be a modifiable lvalue. The type of the operand shall be an arithmetic type other than cv bool, or a pointer to a complete object type. The value of the operand object is modified by adding 1 to it. The value computation of the ++ expression is sequenced before the modification of the operand object. With respect to an indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation. [ Note: Therefore, a function call shall not intervene between the lvalue-to-rvalue conversion and the side effect associated with any single postfix ++ operator. — end note ] The result is a prvalue. The type of the result is the cv-unqualified version of the type of the operand. If the operand is a bit-field that cannot represent the incremented value, the resulting value of the bit-field is implementation-defined. See also [expr.add] and [expr.ass].
The operand of postfix -- is decremented analogously to the postfix ++ operator. [ Note: For prefix increment and decrement, see [expr.pre.incr]. — end note ]
8.2.7 Dynamic cast [expr.dynamic.cast]The result of the expression dynamic_cast<T>(v) is the result of converting the expression v to type T. T shall be a pointer or reference to a complete class type, or “pointer to cv void”. The dynamic_cast operator shall not cast away constness.
If T is a pointer type, v shall be a prvalue of a pointer to complete class type, and the result is a prvalue of type T. If T is an lvalue reference type, v shall be an lvalue of a complete class type, and the result is an lvalue of the type referred to by T. If T is an rvalue reference type, v shall be a glvalue having a complete class type, and the result is an xvalue of the type referred to by T.
If the type of v is the same as T, or it is the same as T except that the class object type in T is more cv-qualified than the class object type in v, the result is v (converted if necessary).
If the value of v is a null pointer value in the pointer case, the result is the null pointer value of type T.
If T is “pointer to cv1 B” and v has type “pointer to cv2 D” such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v. Similarly, if T is “reference to cv1 B” and v has type cv2 D such that B is a base class of D, the result is the unique B subobject of the D object referred to by v.69 In both the pointer and reference cases, the program is ill-formed if cv2 has greater cv-qualification than cv1 or if B is an inaccessible or ambiguous base class of D. [ Example:
struct B { };
struct D : B { };
void foo(D* dp) {
B* bp = dynamic_cast<B*>(dp); }
— end example ]
If T is “pointer to cv void”, then the result is a pointer to the most derived object pointed to by v. Otherwise, a runtime check is applied to see if the object pointed or referred to by v can be converted to the type pointed or referred to by T.
If C is the class type to which T points or refers, the runtime check logically executes as follows:
If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v the result points (refers) to that C object.
Otherwise, if v points (refers) to a public base class subobject of the most derived object, and the type of the most derived object has a base class, of type C, that is unambiguous and public, the result points (refers) to the C subobject of the most derived object.
Otherwise, the runtime check fails.
The value of a failed cast to pointer type is the null pointer value of the required result type. A failed cast to reference type throws an exception of a type that would match a handler of type std::bad_cast.
class A { virtual void f(); };
class B { virtual void g(); };
class D : public virtual A, private B { };
void g() {
D d;
B* bp = (B*)&d; A* ap = &d; D& dr = dynamic_cast<D&>(*bp); ap = dynamic_cast<A*>(bp); bp = dynamic_cast<B*>(ap); ap = dynamic_cast<A*>(&d); bp = dynamic_cast<B*>(&d); }
class E : public D, public B { };
class F : public E, public D { };
void h() {
F f;
A* ap = &f; D* dp = dynamic_cast<D*>(ap); E* ep = (E*)ap; E* ep1 = dynamic_cast<E*>(ap); }
— end example ] [ Note: [class.cdtor] describes the behavior of a dynamic_cast applied to an object under construction or destruction. — end note ]
8.2.8 Type identification [expr.typeid]The result of a typeid expression is an lvalue of static type const std::type_info and dynamic type const std::type_info or const name where name is an implementation-defined class publicly derived from std::type_info which preserves the behavior described in [type.info].70 The lifetime of the object referred to by the lvalue extends to the end of the program. Whether or not the destructor is called for the std::type_info object at the end of the program is unspecified.
When typeid is applied to a glvalue expression whose type is a polymorphic class type, the result refers to a std::type_info object representing the type of the most derived object (that is, the dynamic type) to which the glvalue refers. If the glvalue expression is obtained by applying the unary * operator to a pointer71 and the pointer is a null pointer value, the typeid expression throws an exception of a type that would match a handler of type std::bad_typeid exception.
When typeid is applied to a type-id, the result refers to a std::type_info object representing the type of the type-id. If the type of the type-id is a reference to a possibly cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified referenced type. If the type of the type-id is a class type or a reference to a class type, the class shall be completely-defined.
If the type of the expression or type-id is a cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified type. [ Example:
class D { /* ... */ };
D d1;
const D d2;
typeid(d1) == typeid(d2); typeid(D) == typeid(const D); typeid(D) == typeid(d2); typeid(D) == typeid(const D&);
— end example ]
If the header <typeinfo> is not included prior to a use of typeid, the program is ill-formed.
[ Note: [class.cdtor] describes the behavior of typeid applied to an object under construction or destruction. — end note ]
8.2.9 Static cast [expr.static.cast]The result of the expression static_cast<T>(v) is the result of converting the expression v to type T. If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue. The static_cast operator shall not cast away constness.
An lvalue of type “cv1 B”, where B is a class type, can be cast to type “reference to cv2 D”, where D is a class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed. An xvalue of type “cv1 B” can be cast to type “rvalue reference to cv2 D” with the same constraints as for an lvalue of type “cv1 B”. If the object of type “cv1 B” is actually a base class subobject of an object of type D, the result refers to the enclosing object of type D. Otherwise, the behavior is undefined. [ Example:
struct B { };
struct D : public B { };
D d;
B &br = d;
static_cast<D&>(br);
— end example ]
An lvalue of type “cv1 T1” can be cast to type “rvalue reference to cv2 T2” if “cv2 T2” is reference-compatible with “cv1 T1”. If the value is not a bit-field, the result refers to the object or the specified base class subobject thereof; otherwise, the lvalue-to-rvalue conversion is applied to the bit-field and the resulting prvalue is used as the expression of the static_cast for the remainder of this section. If T2 is an inaccessible or ambiguous base class of T1, a program that necessitates such a cast is ill-formed.
An expression e can be explicitly converted to a type T if there is an implicit conversion sequence from e to T, or if overload resolution for a direct-initialization of an object or reference of type T from e would find at least one viable function ([over.match.viable]). If T is a reference type, the effect is the same as performing the declaration and initialization
T t(e);
for some invented temporary variable t ([dcl.init]) and then using the temporary variable as the result of the conversion. Otherwise, the result object is direct-initialized from e. [ Note: The conversion is ill-formed when attempting to convert an expression of class type to an inaccessible or ambiguous base class. — end note ]
Otherwise, the static_cast shall perform one of the conversions listed below. No other conversion shall be performed explicitly using a static_cast.
Any expression can be explicitly converted to type cv void, in which case it becomes a discarded-value expression. [ Note: However, if the value is in a temporary object, the destructor for that object is not executed until the usual time, and the value of the object is preserved for the purpose of executing the destructor. — end note ]
A value of a scoped enumeration type can be explicitly converted to an integral type. When that type is cv bool, the resulting value is false if the original value is zero and true for all other values. For the remaining integral types, the value is unchanged if the original value can be represented by the specified type. Otherwise, the resulting value is unspecified. A value of a scoped enumeration type can also be explicitly converted to a floating-point type; the result is the same as that of converting from the original value to the floating-point type.
A value of integral or enumeration type can be explicitly converted to a complete enumeration type. The value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]). Otherwise, the behavior is undefined. A value of floating-point type can also be explicitly converted to an enumeration type. The resulting value is the same as converting the original value to the underlying type of the enumeration ([conv.fpint]), and subsequently to the enumeration type.
A prvalue of type “pointer to cv1 B”, where B is a class type, can be converted to a prvalue of type “pointer to cv2 D”, where D is a class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed. The null pointer value is converted to the null pointer value of the destination type. If the prvalue of type “pointer to cv1 B” points to a B that is actually a subobject of an object of type D, the resulting pointer points to the enclosing object of type D. Otherwise, the behavior is undefined.
A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue of type “pointer to member of B of type cv2 T”, where B is a base class of D, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.72 If no valid standard conversion from “pointer to member of B of type T” to “pointer to member of D of type T” exists ([conv.mem]), the program is ill-formed. The null member pointer value is converted to the null member pointer value of the destination type. If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member. Otherwise, the behavior is undefined. [ Note: Although class B need not contain the original member, the dynamic type of the object with which indirection through the pointer to member is performed must contain the original member; see [expr.mptr.oper]. — end note ]
A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. If the original pointer value represents the address A of a byte in memory and A does not satisfy the alignment requirement of T, then the resulting pointer value is unspecified. Otherwise, if the original pointer value points to an object a, and there is an object b of type T (ignoring cv-qualification) that is pointer-interconvertible with a, the result is a pointer to b. Otherwise, the pointer value is unchanged by the conversion. [ Example:
T* p1 = new T; const T* p2 = static_cast<const T*>(static_cast<void*>(p1)); bool b = p1 == p2;
— end example ]
8.2.10 Reinterpret cast [expr.reinterpret.cast]The result of the expression reinterpret_cast<T>(v) is the result of converting the expression v to type T. If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v. Conversions that can be performed explicitly using reinterpret_cast are listed below. No other conversion can be performed explicitly using reinterpret_cast.
The reinterpret_cast operator shall not cast away constness. An expression of integral, enumeration, pointer, or pointer-to-member type can be explicitly converted to its own type; such a cast yields the value of its operand.
[ Note: The mapping performed by reinterpret_cast might, or might not, produce a representation different from the original value. — end note ]
A pointer can be explicitly converted to any integral type large enough to hold it. The mapping function is implementation-defined. [ Note: It is intended to be unsurprising to those who know the addressing structure of the underlying machine. — end note ] A value of type std::nullptr_t can be converted to an integral type; the conversion has the same meaning and validity as a conversion of (void*)0 to the integral type. [ Note: A reinterpret_cast cannot be used to convert a value of any type to the type std::nullptr_t. — end note ]
A value of integral type or enumeration type can be explicitly converted to a pointer. A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined. [ Note: Except as described in [basic.stc.dynamic.safety], the result of such a conversion will not be a safely-derived pointer value. — end note ]
A function pointer can be explicitly converted to a function pointer of a different type. [ Note: The effect of calling a function through a pointer to a function type ([dcl.fct]) that is not the same as the type used in the definition of the function is undefined. — end note ] Except that converting a prvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. [ Note: See also [conv.ptr] for more details of pointer conversions. — end note ]
An object pointer can be explicitly converted to an object pointer of a different type.73 When a prvalue v of object pointer type is converted to the object pointer type “pointer to cv T”, the result is static_cast<cv T*>(static_cast<cv void*>(v)). [ Note: Converting a prvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are object types and where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value. — end note ]
Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning of such a conversion is implementation-defined, except that if an implementation supports conversions in both directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification, shall yield the original pointer value.
The null pointer value is converted to the null pointer value of the destination type. [ Note: A null pointer constant of type std::nullptr_t cannot be converted to a pointer type, and a null pointer constant of integral type is not necessarily converted to a null pointer value. — end note ]
A prvalue of type “pointer to member of X of type T1” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2” if T1 and T2 are both function types or both object types.74 The null member pointer value is converted to the null member pointer value of the destination type. The result of this conversion is unspecified, except in the following cases:
converting a prvalue of type “pointer to member function” to a different pointer to member function type and back to its original type yields the original pointer to member value.
converting a prvalue of type “pointer to data member of X of type T1” to the type “pointer to data member of Y of type T2” (where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer to member value.
A glvalue expression of type T1 can be cast to the type “reference to T2” if an expression of type “pointer to T1” can be explicitly converted to the type “pointer to T2” using a reinterpret_cast. The result refers to the same object as the source glvalue, but with the specified type. [ Note: That is, for lvalues, a reference cast reinterpret_cast<T&>(x) has the same effect as the conversion *reinterpret_cast<T*>(&x) with the built-in & and * operators (and similarly for reinterpret_cast<T&&>(x)). — end note ] No temporary is created, no copy is made, and constructors or conversion functions are not called.75
8.2.11 Const cast [expr.const.cast]The result of the expression const_cast<T>(v) is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.
[ Note: Subject to the restrictions in this section, an expression may be cast to its own type using a const_cast operator. — end note ]
For two similar types T1 and T2, a prvalue of type T1 may be explicitly converted to the type T2 using a const_cast. The result of a const_cast refers to the original entity. [ Example:
typedef int *A[3]; typedef const int *const CA[3];
CA &&r = A{}; A &&r1 = const_cast<A>(CA{}); A &&r2 = const_cast<A&&>(CA{});
— end example ]
For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:
an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.
The result of a reference const_cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion otherwise.
[ Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier76 may produce undefined behavior ([dcl.type.cv]). — end note ]
A conversion from a type T1 to a type T2 casts away constness if T1 and T2 are different, there is a cv-decomposition of T1 yielding n such that T2 has a cv-decomposition of the form cv20 P20 cv21 P21 ⋯ cv2n−1 P2n−1 cv2n U2, and there is no qualification conversion that converts T1 to cv20 P10 cv21 P11 ⋯ cv2n−1 P1n−1 cv2n U1.
Casting from an lvalue of type T1 to an lvalue of type T2 using an lvalue reference cast or casting from an expression of type T1 to an xvalue of type T2 using an rvalue reference cast casts away constness if a cast from a prvalue of type “pointer to T1” to the type “pointer to T2” casts away constness.
[ Note: Some conversions which involve only changes in cv-qualification cannot be done using const_cast. For instance, conversions between pointers to functions are not covered because such conversions lead to values whose use causes undefined behavior. For the same reasons, conversions between pointers to member functions, and in particular, the conversion from a pointer to a const member function to a pointer to a non-const member function, are not covered. — end note ]
8.3 Unary expressions [expr.unary] 8.3.1 Unary operators [expr.unary.op]The unary * operator performs indirection: the expression to which it is applied shall be a pointer to an object type, or a pointer to a function type and the result is an lvalue referring to the object or function to which the expression points. If the type of the expression is “pointer to T”, the type of the result is “T”. [ Note: Indirection through a pointer to an incomplete type (other than cv void) is valid. The lvalue thus obtained can be used in limited ways (to initialize a reference, for example); this lvalue must not be converted to a prvalue, see [conv.lval]. — end note ]
The result of each of the following unary operators is a prvalue.
The result of the unary & operator is a pointer to its operand. The operand shall be an lvalue or a qualified-id. If the operand is a qualified-id naming a non-static or variant member m of some class C with type T, the result has type “pointer to member of class C of type T” and is a prvalue designating C::m. Otherwise, if the type of the expression is T, the result has type “pointer to T” and is a prvalue that is the address of the designated object ([intro.memory]) or a pointer to the designated function. [ Note: In particular, the address of an object of type “cv T” is “pointer to cv T”, with the same cv-qualification. — end note ] For purposes of pointer arithmetic ([expr.add]) and comparison ([expr.rel], [expr.eq]), an object that is not an array element whose address is taken in this way is considered to belong to an array with one element of type T. [ Example:
struct A { int i; };
struct B : A { };
... &B::i ... int a;
int* p1 = &a;
int* p2 = p1 + 1; bool b = p2 > p1;
— end example ] [ Note: A pointer to member formed from a mutable non-static data member ([dcl.stc]) does not reflect the mutable specifier associated with the non-static data member. — end note ]
A pointer to member is only formed when an explicit & is used and its operand is a qualified-id not enclosed in parentheses. [ Note: That is, the expression &(qualified-id), where the qualified-id is enclosed in parentheses, does not form an expression of type “pointer to member”. Neither does qualified-id, because there is no implicit conversion from a qualified-id for a non-static member function to the type “pointer to member function” as there is from an lvalue of function type to the type “pointer to function” ([conv.func]). Nor is &unqualified-id a pointer to member, even within the scope of the unqualified-id's class. — end note ]
If & is applied to an lvalue of incomplete class type and the complete type declares operator&(), it is unspecified whether the operator has the built-in meaning or the operator function is called. The operand of & shall not be a bit-field.
The address of an overloaded function can be taken only in a context that uniquely determines which version of the overloaded function is referred to (see [over.over]). [ Note: Since the context might determine whether the operand is a static or non-static member function, the context can also affect whether the expression has type “pointer to function” or “pointer to member function”. — end note ]
The operand of the unary + operator shall have arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument. Integral promotion is performed on integral or enumeration operands. The type of the result is the type of the promoted operand.
The operand of the unary - operator shall have arithmetic or unscoped enumeration type and the result is the negation of its operand. Integral promotion is performed on integral or enumeration operands. The negative of an unsigned quantity is computed by subtracting its value from 2n, where n is the number of bits in the promoted operand. The type of the result is the type of the promoted operand.
The operand of the logical negation operator ! is contextually converted to bool; its value is true if the converted operand is false and false otherwise. The type of the result is bool.
The operand of ~ shall have integral or unscoped enumeration type; the result is the ones' complement of its operand. Integral promotions are performed. The type of the result is the type of the promoted operand. There is an ambiguity in the grammar when ~ is followed by a class-name or decltype-specifier. The ambiguity is resolved by treating ~ as the unary complement operator rather than as the start of an unqualified-id naming a destructor. [ Note: Because the grammar does not permit an operator to follow the ., ->, or :: tokens, a ~ followed by a class-name or decltype-specifier in a member access expression or qualified-id is unambiguously parsed as a destructor name. — end note ]
8.3.2 Increment and decrement [expr.pre.incr]The operand of prefix ++ is modified by adding 1. The operand shall be a modifiable lvalue. The type of the operand shall be an arithmetic type other than cv bool, or a pointer to a completely-defined object type. The result is the updated operand; it is an lvalue, and it is a bit-field if the operand is a bit-field. The expression ++x is equivalent to x+=1. [ Note: See the discussions of addition and assignment operators for information on conversions. — end note ]
The operand of prefix -- is modified by subtracting 1. The requirements on the operand of prefix -- and the properties of its result are otherwise the same as those of prefix ++. [ Note: For postfix increment and decrement, see [expr.post.incr]. — end note ]
8.3.3 Sizeof [expr.sizeof]The sizeof operator yields the number of bytes in the object representation of its operand. The operand is either an expression, which is an unevaluated operand, or a parenthesized type-id. The sizeof operator shall not be applied to an expression that has function or incomplete type, to the parenthesized name of such types, or to a glvalue that designates a bit-field. sizeof(char), sizeof(signed char) and sizeof(unsigned char) are 1. The result of sizeof applied to any other fundamental type is implementation-defined. [ Note: In particular, sizeof(bool), sizeof(char16_t), sizeof(char32_t), and sizeof(wchar_t) are implementation-defined.77 — end note ] [ Note: See [intro.memory] for the definition of byte and [basic.types] for the definition of object representation. — end note ]
When applied to a reference or a reference type, the result is the size of the referenced type. When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array. The size of a most derived class shall be greater than zero. The result of applying sizeof to a base class subobject is the size of the base class type.78 When applied to an array, the result is the total number of bytes in the array. This implies that the size of an array of n elements is n times the size of an element.
The sizeof operator can be applied to a pointer to a function, but shall not be applied directly to a function.
The identifier in a sizeof... expression shall name a parameter pack. The sizeof... operator yields the number of arguments provided for the parameter pack identifier. A sizeof... expression is a pack expansion. [ Example:
template<class... Types>
struct count {
static const std::size_t value = sizeof...(Types);
};
— end example ]
The result of sizeof and sizeof... is a constant of type std::size_t. [ Note: std::size_t is defined in the standard header <cstddef> ([cstddef.syn], [support.types.layout]). — end note ]
8.3.4 New [expr.new]The new-type-id in a new-expression is the longest possible sequence of new-declarators. [ Note: This prevents ambiguities between the declarator operators &, &&, *, and [] and their expression counterparts. — end note ] [ Example:
new int * i;
The * is the pointer declarator and not the multiplication operator. — end example ]
[ Note: Parentheses in a new-type-id of a new-expression can have surprising effects. [ Example:
new int(*[10])();
is ill-formed because the binding is
(new int) (*[10])();
Instead, the explicitly parenthesized version of the new operator can be used to create objects of compound types:
new (int (*[10])());
allocates an array of 10 pointers to functions (taking no argument and returning int). — end example ] — end note ]
The expression in a noptr-new-declarator is erroneous if:
the expression is of non-class type and its value before converting to std::size_t is less than zero;
the expression is of class type and its value before application of the second standard conversion ([over.ics.user])79 is less than zero;
its value is such that the size of the allocated object would exceed the implementation-defined limit; or
the new-initializer is a braced-init-list and the number of array elements for which initializers are provided (including the terminating '\0' in a string literal) exceeds the number of elements to initialize.
If the expression is erroneous after converting to std::size_t:
if the expression is a core constant expression, the program is ill-formed;
otherwise, an allocation function is not called; instead
if the allocation function that would have been called has a non-throwing exception specification ([except.spec]), the value of the new-expression is the null pointer value of the required result type;
otherwise, the new-expression terminates by throwing an exception of a type that would match a handler ([except.handle]) of type std::bad_array_new_length.
When the value of the expression is zero, the allocation function is called to allocate an array with no elements.
A new-expression may obtain storage for the object by calling an allocation function ([basic.stc.dynamic.allocation]). If the new-expression terminates by throwing an exception, it may release storage by calling a deallocation function. If the allocated type is a non-array type, the allocation function's name is operator new and the deallocation function's name is operator delete. If the allocated type is an array type, the allocation function's name is operator new[] and the deallocation function's name is operator delete[]. [ Note: An implementation shall provide default definitions for the global allocation functions ([basic.stc.dynamic], [new.delete.single], [new.delete.array]). A C++ program can provide alternative definitions of these functions ([replacement.functions]) and/or class-specific versions ([class.free]). The set of allocation and deallocation functions that may be called by a new-expression may include functions that do not perform allocation or deallocation; for example, see [new.delete.placement]. — end note ]
If the new-expression begins with a unary :: operator, the allocation function's name is looked up in the global scope. Otherwise, if the allocated type is a class type T or array thereof, the allocation function's name is looked up in the scope of T. If this lookup fails to find the name, or if the allocated type is not a class type, the allocation function's name is looked up in the global scope.
An implementation is allowed to omit a call to a replaceable global allocation function ([new.delete.single], [new.delete.array]). When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression. The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended:
the evaluation of e1 is sequenced before the evaluation of e2, and
e2 is evaluated whenever e1 obtains storage, and
both e1 and e2 invoke the same replaceable global allocation function, and
if the allocation function invoked by e1 and e2 is throwing, any exceptions thrown in the evaluation of either e1 or e2 would be first caught in the same handler, and
the pointer values produced by e1 and e2 are operands to evaluated delete-expressions, and
the evaluation of e2 is sequenced before the evaluation of the delete-expression whose operand is the pointer value produced by e1.
[ Example:
void mergeable(int x) {
std::unique_ptr<char[]> a{new (std::nothrow) char[8]};
std::unique_ptr<char[]> b{new (std::nothrow) char[8]};
std::unique_ptr<char[]> c{new (std::nothrow) char[x]};
g(a.get(), b.get(), c.get());
}
void unmergeable(int x) {
std::unique_ptr<char[]> a{new char[8]};
try {
std::unique_ptr<char[]> b{new char[x]};
} catch (const std::bad_alloc& e) {
std::cerr << "Allocation failed: " << e.what() << std::endl;
throw;
}
}
— end example ]
When a new-expression calls an allocation function and that allocation has not been extended, the new-expression passes the amount of space requested to the allocation function as the first argument of type std::size_t. That argument shall be no less than the size of the object being created; it may be greater than the size of the object being created only if the object is an array. For arrays of char, unsigned char, and std::byte, the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement of any object type whose size is no greater than the size of the array being created. [ Note: Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating character arrays into which objects of other types will later be placed. — end note ]
When a new-expression calls an allocation function and that allocation has been extended, the size argument to the allocation call shall be no greater than the sum of the sizes for the omitted calls as specified above, plus the size for the extended call had it not been extended, plus any padding necessary to align the allocated objects within the allocated memory.
Overload resolution is performed on a function call created by assembling an argument list. The first argument is the amount of space requested, and has type std::size_t. If the type of the allocated object has new-extended alignment, the next argument is the type's alignment, and has type std::align_val_t. If the new-placement syntax is used, the initializer-clauses in its expression-list are the succeeding arguments. If no matching function is found and the allocated object type has new-extended alignment, the alignment argument is removed from the argument list, and overload resolution is performed again.
[ Example:
new T results in one of the following calls:
operator new(sizeof(T)) operator new(sizeof(T), std::align_val_t(alignof(T)))
new(2,f) T results in one of the following calls:
operator new(sizeof(T), 2, f) operator new(sizeof(T), std::align_val_t(alignof(T)), 2, f)
new T[5] results in one of the following calls:
operator new[](sizeof(T) * 5 + x) operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)))
new(2,f) T[5] results in one of the following calls:
operator new[](sizeof(T) * 5 + x, 2, f) operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)), 2, f)
Here, each instance of x is a non-negative unspecified value representing array allocation overhead; the result of the new-expression will be offset by this amount from the value returned by operator new[]. This overhead may be applied in all array new-expressions, including those referencing the library function operator new[](std::size_t, void*) and other placement allocation functions. The amount of overhead may vary from one invocation of new to another. — end example ]
[ Note: Unless an allocation function has a non-throwing exception specification, it indicates failure to allocate storage by throwing a std::bad_alloc exception ([basic.stc.dynamic.allocation], Clause [except], [bad.alloc]); it returns a non-null pointer otherwise. If the allocation function has a non-throwing exception specification, it returns null to indicate failure to allocate storage and a non-null pointer otherwise. — end note ] If the allocation function is a non-allocating form ([new.delete.placement]) that returns null, the behavior is undefined. Otherwise, if the allocation function returns null, initialization shall not be done, the deallocation function shall not be called, and the value of the new-expression shall be null.
[ Note: When the allocation function returns a value other than null, it must be a pointer to a block of storage in which space for the object has been reserved. The block of storage is assumed to be appropriately aligned and of the requested size. The address of the created object will not necessarily be the same as that of the block if the object is an array. — end note ]
A new-expression that creates an object of type T initializes that object as follows:
If the new-initializer is omitted, the object is default-initialized ([dcl.init]). [ Note: If no initialization is performed, the object has an indeterminate value. — end note ]
Otherwise, the new-initializer is interpreted according to the initialization rules of [dcl.init] for direct-initialization.
The invocation of the allocation function is sequenced before the evaluations of expressions in the new-initializer. Initialization of the allocated object is sequenced before the value computation of the new-expression.
If any part of the object initialization described above80 terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression. If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed. [ Note: This is appropriate when the called allocation function does not allocate memory; otherwise, it is likely to result in a memory leak. — end note ]
If the new-expression begins with a unary :: operator, the deallocation function's name is looked up in the global scope. Otherwise, if the allocated type is a class type T or an array thereof, the deallocation function's name is looked up in the scope of T. If this lookup fails to find the name, or if the allocated type is not a class type or array thereof, the deallocation function's name is looked up in the global scope.
A declaration of a placement deallocation function matches the declaration of a placement allocation function if it has the same number of parameters and, after parameter transformations ([dcl.fct]), all parameter types except the first are identical. If the lookup finds a single matching deallocation function, that function will be called; otherwise, no deallocation function will be called. If the lookup finds a usual deallocation function with a parameter of type std::size_t and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed. For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function ([expr.delete]) [ Example:
struct S {
static void* operator new(std::size_t, std::size_t);
static void operator delete(void*, std::size_t);
};
S* p = new (0) S;
— end example ]
If a new-expression calls a deallocation function, it passes the value returned from the allocation function call as the first argument of type void*. If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax. If the implementation is allowed to make a copy of any argument as part of the call to the allocation function, it is allowed to make a copy (of the same original value) as part of the call to the deallocation function or to reuse the copy made as part of the call to the allocation function. If the copy is elided in one place, it need not be elided in the other.
8.3.5 Delete [expr.delete]If the operand has a class type, the operand is converted to a pointer type by calling the above-mentioned conversion function, and the converted operand is used in place of the original operand for the remainder of this section. In the first alternative (delete object), the value of the operand of delete may be a null pointer value, a pointer to a non-array object created by a previous new-expression, or a pointer to a subobject representing a base class of such an object. If not, the behavior is undefined. In the second alternative (delete array), the value of the operand of delete may be a null pointer value or a pointer value that resulted from a previous array new-expression.83 If not, the behavior is undefined. [ Note: This means that the syntax of the delete-expression must match the type of the object allocated by new, not the syntax of the new-expression. — end note ] [ Note: A pointer to a const type can be the operand of a delete-expression; it is not necessary to cast away the constness of the pointer expression before it is used as the operand of the delete-expression. — end note ]
In the first alternative (delete object), if the static type of the object to be deleted is different from its dynamic type, the static type shall be a base class of the dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined. In the second alternative (delete array) if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.
If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.
If the value of the operand of the delete-expression is not a null pointer value, the delete-expression will invoke the destructor (if any) for the object or the elements of the array being deleted. In the case of an array, the elements will be destroyed in order of decreasing address (that is, in reverse order of the completion of their constructor; see [class.base.init]).
If the value of the operand of the delete-expression is not a null pointer value, then:
If the allocation call for the new-expression for the object to be deleted was not omitted and the allocation was not extended ([expr.new]), the delete-expression shall call a deallocation function. The value returned from the allocation call of the new-expression shall be passed as the first argument to the deallocation function.
Otherwise, if the allocation was extended or was provided by extending the allocation of another new-expression, and the delete-expression for every other pointer value produced by a new-expression that had storage provided by the extended new-expression has been evaluated, the delete-expression shall call a deallocation function. The value returned from the allocation call of the extended new-expression shall be passed as the first argument to the deallocation function.
Otherwise, the delete-expression will not call a deallocation function.
[ Note: The deallocation function is called regardless of whether the destructor for the object or some element of the array throws an exception. — end note ] If the value of the operand of the delete-expression is a null pointer value, it is unspecified whether a deallocation function will be called as described above.
When the keyword delete in a delete-expression is preceded by the unary :: operator, the deallocation function's name is looked up in global scope. Otherwise, the lookup considers class-specific deallocation functions ([class.free]). If no class-specific deallocation function is found, the deallocation function's name is looked up in global scope.
If deallocation function lookup finds more than one usual deallocation function, the function to be called is selected as follows:
If the type has new-extended alignment, a function with a parameter of type std::align_val_t is preferred; otherwise a function without such a parameter is preferred. If exactly one preferred function is found, that function is selected and the selection process terminates. If more than one preferred function is found, all non-preferred functions are eliminated from further consideration.
If the deallocation functions have class scope, the one without a parameter of type std::size_t is selected.
If the type is complete and if, for the second alternative (delete array) only, the operand is a pointer to a class type with a non-trivial destructor or a (possibly multi-dimensional) array thereof, the function with a parameter of type std::size_t is selected.
Otherwise, it is unspecified whether a deallocation function with a parameter of type std::size_t is selected.
When a delete-expression is executed, the selected deallocation function shall be called with the address of the most-derived object in the delete object case, or the address of the object suitably adjusted for the array allocation overhead ([expr.new]) in the delete array case, as its first argument. If a deallocation function with a parameter of type std::align_val_t is used, the alignment of the type of the object to be deleted is passed as the corresponding argument. If a deallocation function with a parameter of type std::size_t is used, the size of the most-derived type, or of the array plus allocation overhead, respectively, is passed as the corresponding argument.84 [ Note: If this results in a call to a usual deallocation function, and either the first argument was not the result of a prior call to a usual allocation function or the second argument was not the corresponding argument in said call, the behavior is undefined ([new.delete.single], [new.delete.array]). — end note ]
Access and ambiguity control are done for both the deallocation function and the destructor ([class.dtor], [class.free]).
8.3.6 Alignof [expr.alignof]An alignof expression yields the alignment requirement of its operand type. The operand shall be a type-id representing a complete object type, or an array thereof, or a reference to one of those types.
The result is an integral constant of type std::size_t.
When alignof is applied to a reference type, the result is the alignment of the referenced type. When alignof is applied to an array type, the result is the alignment of the element type.
8.4 Explicit type conversion (cast notation) [expr.cast]The result of the expression (T) cast-expression is of type T. The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue if T is an rvalue reference to object type; otherwise the result is a prvalue. [ Note: If T is a non-class type that is cv-qualified, the cv-qualifiers are discarded when determining the type of the resulting prvalue; see Clause [expr]. — end note ]
Any type conversion not mentioned below and not explicitly defined by the user ([class.conv]) is ill-formed.
The conversions performed by
a const_cast,
a static_cast,
a static_cast followed by a const_cast,
a reinterpret_cast, or
a reinterpret_cast followed by a const_cast,
can be performed using the cast notation of explicit type conversion. The same semantic restrictions and behaviors apply, with the exception that in performing a static_cast in the following situations the conversion is valid even if the base class is inaccessible:
a pointer to an object of derived class type or an lvalue or rvalue of derived class type may be explicitly converted to a pointer or reference to an unambiguous base class type, respectively;
a pointer to member of derived class type may be explicitly converted to a pointer to member of an unambiguous non-virtual base class type;
a pointer to an object of an unambiguous non-virtual base class type, a glvalue of an unambiguous non-virtual base class type, or a pointer to member of an unambiguous non-virtual base class type may be explicitly converted to a pointer, a reference, or a pointer to member of a derived class type, respectively.
If a conversion can be interpreted in more than one of the ways listed above, the interpretation that appears first in the list is used, even if a cast resulting from that interpretation is ill-formed. If a conversion can be interpreted in more than one way as a static_cast followed by a const_cast, the conversion is ill-formed. [ Example:
struct A { };
struct I1 : A { };
struct I2 : A { };
struct D : I1, I2 { };
A* foo( D* p ) {
return (A*)( p ); }
— end example ]
The operand of a cast using the cast notation can be a prvalue of type “pointer to incomplete class type”. The destination type of a cast using the cast notation can be “pointer to incomplete class type”. If both the operand and destination types are class types and one or both are incomplete, it is unspecified whether the static_cast or the reinterpret_cast interpretation is used, even if there is an inheritance relationship between the two classes. [ Note: For example, if the classes were defined later in the translation unit, a multi-pass compiler would be permitted to interpret a cast between pointers to the classes as if the class types were complete at the point of the cast. — end note ]
8.5 Pointer-to-member operators [expr.mptr.oper]The binary operator .* binds its second operand, which shall be of type “pointer to member of T” to its first operand, which shall be a glvalue of class T or of a class of which T is an unambiguous and accessible base class. The result is an object or a function of the type specified by the second operand.
The binary operator ->* binds its second operand, which shall be of type “pointer to member of T” to its first operand, which shall be of type “pointer to U” where U is either T or a class of which T is an unambiguous and accessible base class. The expression E1->*E2 is converted into the equivalent form (*(E1)).*E2.
Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression. If the dynamic type of E1 does not contain the member to which E2 refers, the behavior is undefined. Otherwise, the expression E1 is sequenced before the expression E2.
The restrictions on cv-qualification, and the manner in which the cv-qualifiers of the operands are combined to produce the cv-qualifiers of the result, are the same as the rules for E1.E2 given in [expr.ref]. [ Note: It is not possible to use a pointer to member that refers to a mutable member to modify a const class object. For example,
struct S {
S() : i(0) { }
mutable int i;
};
void f()
{
const S cs;
int S::* pm = &S::i; cs.*pm = 88; }
— end note ]
If the result of .* or ->* is a function, then that result can be used only as the operand for the function call operator (). [ Example:
(ptr_to_obj->*ptr_to_mfct)(10);
calls the member function denoted by ptr_to_mfct for the object pointed to by ptr_to_obj. — end example ] In a .* expression whose object expression is an rvalue, the program is ill-formed if the second operand is a pointer to member function with ref-qualifier &. In a .* expression whose object expression is an lvalue, the program is ill-formed if the second operand is a pointer to member function with ref-qualifier &&. The result of a .* expression whose second operand is a pointer to a data member is an lvalue if the first operand is an lvalue and an xvalue otherwise. The result of a .* expression whose second operand is a pointer to a member function is a prvalue. If the second operand is the null member pointer value, the behavior is undefined.
8.6 Multiplicative operators [expr.mul]The operands of * and / shall have arithmetic or unscoped enumeration type; the operands of % shall have integral or unscoped enumeration type. The usual arithmetic conversions are performed on the operands and determine the type of the result.
The binary * operator indicates multiplication.
The binary / operator yields the quotient, and the binary % operator yields the remainder from the division of the first expression by the second. If the second operand of / or % is zero the behavior is undefined. For integral operands the / operator yields the algebraic quotient with any fractional part discarded;85 if the quotient a/b is representable in the type of the result, (a/b)*b + a%b is equal to a; otherwise, the behavior of both a/b and a%b is undefined.
8.7 Additive operators [expr.add]For subtraction, one of the following shall hold:
both operands have arithmetic or unscoped enumeration type; or
both operands are pointers to cv-qualified or cv-unqualified versions of the same completely-defined object type; or
the left operand is a pointer to a completely-defined object type and the right operand has integral or unscoped enumeration type.
The result of the binary + operator is the sum of the operands. The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first.
When an expression that has integral type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the expression P points to element x[i] of an array object x with n elements,86 the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) element x[i+j] if 0≤i+j≤n; otherwise, the behavior is undefined. Likewise, the expression P - J points to the (possibly-hypothetical) element x[i−j] if 0≤i−j≤n; otherwise, the behavior is undefined.
When two pointers to elements of the same array object are subtracted, the type of the result is an implementation-defined signed integral type; this type shall be the same type that is defined as std::ptrdiff_t in the <cstddef> header ([support.types]). If the expressions P and Q point to, respectively, elements x[i] and x[j] of the same array object x, the expression P - Q has the value i−j; otherwise, the behavior is undefined. [ Note: If the value i−j is not in the range of representable values of type std::ptrdiff_t, the behavior is undefined. — end note ]
For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T and the array element type are not similar, the behavior is undefined. [ Note: In particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type. — end note ]
If the value 0 is added to or subtracted from a null pointer value, the result is a null pointer value. If two null pointer values are subtracted, the result compares equal to the value 0 converted to the type std::ptrdiff_t.
8.8 Shift operators [expr.shift]The value of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are zero-filled. If E1 has an unsigned type, the value of the result is E1×2E2, reduced modulo one more than the maximum value representable in the result type. Otherwise, if E1 has a signed type and non-negative value, and E1×2E2 is representable in the corresponding unsigned type of the result type, then that value, converted to the result type, is the resulting value; otherwise, the behavior is undefined.
The value of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type or if E1 has a signed type and a non-negative value, the value of the result is the integral part of the quotient of E1/2E2. If E1 has a signed type and a negative value, the resulting value is implementation-defined.
The expression E1 is sequenced before the expression E2.
8.9 Relational operators [expr.rel]Comparing unequal pointers to objects87 is defined as follows:
If two pointers point to different elements of the same array, or to subobjects thereof, the pointer to the element with the higher subscript compares greater.
If two pointers point to different non-static data members of the same object, or to subobjects of such members, recursively, the pointer to the later declared member compares greater provided the two members have the same access control and provided their class is not a union.
Otherwise, neither pointer compares greater than the other.
If two operands p and q compare equal, p<=q and p>=q both yield true and p<q and p>q both yield false. Otherwise, if a pointer p compares greater than a pointer q, p>=q, p>q, q<=p, and q<p all yield true and p<=q, p<q, q>=p, and q>p all yield false. Otherwise, the result of each of the operators is unspecified.
If both operands (after conversions) are of arithmetic or enumeration type, each of the operators shall yield true if the specified relationship is true and false if it is false.
8.10 Equality operators [expr.eq]equality-expression: relational-expression equality-expression == relational-expression equality-expression != relational-expression
The == (equal to) and the != (not equal to) operators group left-to-right. The operands shall have arithmetic, enumeration, pointer, or pointer to member type, or type std::nullptr_t. The operators == and != both yield true or false, i.e., a result of type bool. In each case below, the operands shall have the same type after the specified conversions have been applied.
If at least one of the operands is a pointer, pointer conversions, function pointer conversions, and qualification conversions are performed on both operands to bring them to their composite pointer type. Comparing pointers is defined as follows:
If one pointer represents the address of a complete object, and another pointer represents the address one past the last element of a different complete object,88 the result of the comparison is unspecified.
Otherwise, if the pointers are both null, both point to the same function, or both represent the same address, they compare equal.
Otherwise, the pointers compare unequal.
If at least one of the operands is a pointer to member, pointer to member conversions and qualification conversions are performed on both operands to bring them to their composite pointer type. Comparing pointers to members is defined as follows:
If two pointers to members are both the null member pointer value, they compare equal.
If only one of two pointers to members is the null member pointer value, they compare unequal.
If either is a pointer to a virtual member function, the result is unspecified.
If one refers to a member of class C1 and the other refers to a member of a different class C2, where neither is a base class of the other, the result is unspecified. [ Example:
struct A {};
struct B : A { int x; };
struct C : A { int x; };
int A::*bx = (int(A::*))&B::x;
int A::*cx = (int(A::*))&C::x;
bool b1 = (bx == cx);
— end example ]
If both refer to (possibly different) members of the same union, they compare equal.
Otherwise, two pointers to members compare equal if they would refer to the same member of the same most derived object or the same subobject if indirection with a hypothetical object of the associated class type were performed, otherwise they compare unequal. [ Example:
struct B {
int f();
};
struct L : B { };
struct R : B { };
struct D : L, R { };
int (B::*pb)() = &B::f;
int (L::*pl)() = pb;
int (R::*pr)() = pb;
int (D::*pdl)() = pl;
int (D::*pdr)() = pr;
bool x = (pdl == pdr); bool y = (pb == pl);
— end example ]
Two operands of type std::nullptr_t or one operand of type std::nullptr_t and the other a null pointer constant compare equal.
If two operands compare equal, the result is true for the == operator and false for the != operator. If two operands compare unequal, the result is false for the == operator and true for the != operator. Otherwise, the result of each of the operators is unspecified.
If both operands are of arithmetic or enumeration type, the usual arithmetic conversions are performed on both operands; each of the operators shall yield true if the specified relationship is true and false if it is false.
8.16 Conditional operator [expr.cond]conditional-expression: logical-or-expression logical-or-expression ? expression : assignment-expression
Conditional expressions group right-to-left. The first expression is contextually converted to bool. It is evaluated and if it is true, the result of the conditional expression is the value of the second expression, otherwise that of the third expression. Only one of the second and third expressions is evaluated. Every value computation and side effect associated with the first expression is sequenced before every value computation and side effect associated with the second or third expression.
If either the second or the third operand has type void, one of the following shall hold:
The second or the third operand (but not both) is a (possibly parenthesized) throw-expression; the result is of the type and value category of the other. The conditional-expression is a bit-field if that operand is a bit-field.
Both the second and the third operands have type void; the result is of type void and is a prvalue. [ Note: This includes the case where both operands are throw-expressions. — end note ]
Otherwise, if the second and third operand are glvalue bit-fields of the same value category and of types cv1 T and cv2 T, respectively, the operands are considered to be of type cv T for the remainder of this section, where cv is the union of cv1 and cv2.
Otherwise, if the second and third operand have different types and either has (possibly cv-qualified) class type, or if both are glvalues of the same value category and the same type except for cv-qualification, an attempt is made to form an implicit conversion sequence from each of those operands to the type of the other. [ Note: Properties such as access, whether an operand is a bit-field, or whether a conversion function is deleted are ignored for that determination. — end note ] Attempts are made to form an implicit conversion sequence from an operand expression E1 of type T1 to a target type related to the type T2 of the operand expression E2 as follows:
If E2 is an lvalue, the target type is “lvalue reference to T2”, subject to the constraint that in the conversion the reference must bind directly to an lvalue.
If E2 is an xvalue, the target type is “rvalue reference to T2”, subject to the constraint that the reference must bind directly.
If E2 is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type:
if T1 and T2 are the same class type (ignoring cv-qualification), or one is a base class of the other, and T2 is at least as cv-qualified as T1, the target type is T2,
otherwise, the target type is the type that E2 would have after applying the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions.
Using this process, it is determined whether an implicit conversion sequence can be formed from the second operand to the target type determined for the third operand, and vice versa. If both sequences can be formed, or one can be formed but it is the ambiguous conversion sequence, the program is ill-formed. If no conversion sequence can be formed, the operands are left unchanged and further checking is performed as described below. Otherwise, if exactly one conversion sequence can be formed, that conversion is applied to the chosen operand and the converted operand is used in place of the original operand for the remainder of this section. [ Note: The conversion might be ill-formed even if an implicit conversion sequence could be formed. — end note ]
If the second and third operands are glvalues of the same value category and have the same type, the result is of that type and value category and it is a bit-field if the second or the third operand is a bit-field, or if both are bit-fields.
Otherwise, the result is a prvalue. If the second and third operands do not have the same type, and either has (possibly cv-qualified) class type, overload resolution is used to determine the conversions (if any) to be applied to the operands ([over.match.oper], [over.built]). If the overload resolution fails, the program is ill-formed. Otherwise, the conversions thus determined are applied, and the converted operands are used in place of the original operands for the remainder of this section.
Lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the second and third operands. After those conversions, one of the following shall hold:
The second and third operands have the same type; the result is of that type and the result object is initialized using the selected operand.
The second and third operands have arithmetic or enumeration type; the usual arithmetic conversions are performed to bring them to a common type, and the result is of that type.
One or both of the second and third operands have pointer type; pointer conversions, function pointer conversions, and qualification conversions are performed to bring them to their composite pointer type (Clause [expr]). The result is of the composite pointer type.
One or both of the second and third operands have pointer to member type; pointer to member conversions and qualification conversions are performed to bring them to their composite pointer type. The result is of the composite pointer type.
Both the second and third operands have type std::nullptr_t or one has that type and the other is a null pointer constant. The result is of type std::nullptr_t.
throw-expression: throw assignment-expressionopt
Evaluating a throw-expression with an operand throws an exception; the type of the exception object is determined by removing any top-level cv-qualifiers from the static type of the operand and adjusting the type from “array of T” or function type T to “pointer to T”.
A throw-expression with no operand rethrows the currently handled exception. The exception is reactivated with the existing exception object; no new exception object is created. The exception is no longer considered to be caught. [ Example: Code that must be executed because of an exception, but cannot completely handle the exception itself, can be written like this:
try {
} catch (...) { throw; }
— end example ]
8.18 Assignment and compound assignment operators [expr.ass]The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable lvalue as their left operand and return an lvalue referring to the left operand. The result in all cases is a bit-field if the left operand is a bit-field. In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression. The right operand is sequenced before the left operand. With respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation. [ Note: Therefore, a function call shall not intervene between the lvalue-to-rvalue conversion and the side effect associated with any single compound assignment operator. — end note ]
assignment-expression: conditional-expression logical-or-expression assignment-operator initializer-clause throw-expression
assignment-operator: one of = *= /= %= += -= >>= <<= &= ^= |=
In simple assignment (=), the value of the expression replaces that of the object referred to by the left operand.
If the left operand is not of class type, the expression is implicitly converted (Clause [conv]) to the cv-unqualified type of the left operand.
If the left operand is of class type, the class shall be complete. Assignment to objects of a class is defined by the copy/move assignment operator ([class.copy], [over.ass]).
When the left operand of an assignment operator is a bit-field that cannot represent the value of the expression, the resulting value of the bit-field is implementation-defined.
The behavior of an expression of the form E1 op= E2 is equivalent to E1 = E1 op E2 except that E1 is evaluated only once. In += and -=, E1 shall either have arithmetic type or be a pointer to a possibly cv-qualified completely-defined object type. In all other cases, E1 shall have arithmetic type.
If the value being stored in an object is read via another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have the same type, otherwise the behavior is undefined. [ Note: This restriction applies to the relationship between the left and right sides of the assignment operation; it is not a statement about how the target of the assignment may be aliased in general. See [basic.lval]. — end note ]
A braced-init-list may appear on the right-hand side of
an assignment to a scalar, in which case the initializer list shall have at most a single element. The meaning of x = {v}, where T is the scalar type of the expression x, is that of x = T{v}. The meaning of x = {} is x = T{}.
an assignment to an object of class type, in which case the initializer list is passed as the argument to the assignment operator function selected by overload resolution ([over.ass], [over.match]).
[ Example:
complex<double> z;
z = { 1,2 }; z += { 1, 2 }; int a, b;
a = b = { 1 }; a = { 1 } = b;
— end example ]
8.19 Comma operator [expr.comma]The comma operator groups left-to-right.
expression: assignment-expression expression , assignment-expression
A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression. Every value computation and side effect associated with the left expression is sequenced before every value computation and side effect associated with the right expression. The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a bit-field. If the right operand is a temporary expression ([class.temporary]), the result is a temporary expression.
In contexts where comma is given a special meaning, [ Example: in lists of arguments to functions ([expr.call]) and lists of initializers ([dcl.init]) — end example ] the comma operator as described in Clause [expr] can appear only in parentheses. [ Example:
f(a, (t=3, t+2), c);
has three arguments, the second of which has the value 5. — end example ]
8.20 Constant expressions [expr.const]Certain contexts require expressions that satisfy additional requirements as detailed in this subclause; other contexts have different semantics depending on whether or not an expression satisfies these requirements. Expressions that satisfy these requirements, assuming that copy elision is performed, are called constant expressions. [ Note: Constant expressions can be evaluated during translation. — end note ]
constant-expression: conditional-expression
An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine, would evaluate one of the following expressions:
this, except in a constexpr function or a constexpr constructor that is being evaluated as part of e;
an invocation of a function other than a constexpr constructor for a literal class, a constexpr function, or an implicit invocation of a trivial destructor ([class.dtor]) [ Note: Overload resolution is applied as usual — end note ] ;
an invocation of an undefined constexpr function or an undefined constexpr constructor;
an invocation of an instantiated constexpr function or constexpr constructor that fails to satisfy the requirements for a constexpr function or constexpr constructor;
an expression that would exceed the implementation-defined limits;
an operation that would have undefined behavior as specified in Clauses [intro] through [cpp] of this International Standard [ Note: including, for example, signed integer overflow (Clause [expr]), certain pointer arithmetic ([expr.add]), division by zero, or certain shift operations — end note ] ;
an lvalue-to-rvalue conversion unless it is applied to
a non-volatile glvalue of integral or enumeration type that refers to a complete non-volatile const object with a preceding initialization, initialized with a constant expression, or
a non-volatile glvalue that refers to a subobject of a string literal, or
a non-volatile glvalue that refers to a non-volatile object defined with constexpr, or that refers to a non-mutable subobject of such an object, or
a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of e;
an lvalue-to-rvalue conversion that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
an invocation of an implicitly-defined copy/move constructor or copy/move assignment operator for a union whose active member (if any) is mutable, unless the lifetime of the union object began within the evaluation of e;
an assignment expression or invocation of an assignment operator ([class.copy]) that would change the active member of a union;
an id-expression that refers to a variable or data member of reference type unless the reference has a preceding initialization and either
it is initialized with a constant expression or
its lifetime began within the evaluation of e;
in a lambda-expression, a reference to this or to a variable with automatic storage duration defined outside that lambda-expression, where the reference would be an odr-use; [ Example:
void g() {
const int n = 0;
[=] {
constexpr int i = n; constexpr int j = *&n; };
}
— end example ] [ Note: If the odr-use occurs in an invocation of a function call operator of a closure type, it no longer refers to this or to an enclosing automatic variable due to the transformation ([expr.prim.lambda.capture]) of the id-expression into an access of the corresponding data member. [ Example:
auto monad = [](auto v) { return [=] { return v; }; };
auto bind = [](auto m) {
return [=](auto fvm) { return fvm(m()); };
};
static_assert(bind(monad(2))(monad)() == monad(2)());
— end example ] — end note ]
a conversion from type cv void* to a pointer-to-object type;
a dynamic cast;
modification of an object ([expr.ass], [expr.post.incr], [expr.pre.incr]) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of e;
a typeid expression whose operand is a glvalue of a polymorphic class type;
a relational or equality operator where the result is unspecified; or
If e satisfies the constraints of a core constant expression, but evaluation of e would evaluate an operation that has undefined behavior as specified in Clauses [library] through [thread] of this International Standard, it is unspecified whether e is a core constant expression.
[ Example:
int x; struct A {
constexpr A(bool b) : m(b?42:x) { }
int m;
};
constexpr int v = A(true).m;
constexpr int w = A(false).m;
constexpr int f1(int k) {
constexpr int x = k; return x;
}
constexpr int f2(int k) {
int x = k; return x;
}
constexpr int incr(int &n) {
return ++n;
}
constexpr int g(int k) {
constexpr int x = incr(k); return x;
}
constexpr int h(int k) {
int x = incr(k); return x;
}
constexpr int y = h(1);
— end example ]
An integral constant expression is an expression of integral or unscoped enumeration type, implicitly converted to a prvalue, where the converted expression is a core constant expression. [ Note: Such expressions may be used as bit-field lengths, as enumerator initializers if the underlying type is not fixed ([dcl.enum]), and as alignments. — end note ]
A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only
user-defined conversions,
integral conversions other than narrowing conversions,
null pointer conversions from std::nullptr_t,
null member pointer conversions from std::nullptr_t, and
and where the reference binding (if any) binds directly. [ Note: Such expressions may be used in new expressions, as case expressions, as enumerator initializers if the underlying type is fixed, as array bounds, and as non-type template arguments. — end note ] A contextually converted constant expression of type bool is an expression, contextually converted to bool, where the converted expression is a constant expression and the conversion sequence contains only the conversions above.
A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:
if the value is an object of class type, each non-static data member of reference type refers to an entity that is a permitted result of a constant expression,
if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object ([expr.add]), the address of a function, or a null pointer value, and
if the value is an object of class or array type, each subobject satisfies these constraints for the value.
An entity is a permitted result of a constant expression if it is an object with static storage duration that is either not a temporary object or is a temporary object whose value satisfies the above constraints, or it is a function.
[ Note: Since this International Standard imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution.89 [ Example:
bool f() {
char array[1 + int(1 + 0.2 - 0.1 - 0.1)]; int size = 1 + int(1 + 0.2 - 0.1 - 0.1); return sizeof(array) == size;
}
It is unspecified whether the value of f() will be true or false. — end example ] — end note ]
If an expression of literal class type is used in a context where an integral constant expression is required, then that expression is contextually implicitly converted to an integral or unscoped enumeration type and the selected conversion function shall be constexpr. [ Example:
struct A {
constexpr A(int i) : val(i) { }
constexpr operator int() const { return val; }
constexpr operator long() const { return 43; }
private:
int val;
};
template<int> struct X { };
constexpr A a = 42;
X<a> x; int ary[a];
— end example ]
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