A declaration occurs in a scope ([basic.scope]); the scope rules are summarized in [basic.lookup]. A declaration that declares a function or defines a class, namespace, template, or function also has one or more scopes nested within it. These nested scopes, in turn, can have declarations nested within them. Unless otherwise stated, utterances in Clause [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.
In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class (Clause [class]) or enumeration ([dcl.enum]), that is, when the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key ([class.name]), or an enum-specifier. In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in these specifiers are among the names being declared by the declaration (as class-names, enum-names, or enumerators, depending on the syntax). In such cases, the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration. [ Example:
enum { }; typedef class { };
— end example ]
In a static_assert-declaration the constant-expression shall be a constant expression ([expr.const]) that can be contextually converted to bool (Clause [conv]). If the value of the expression when so converted is true, the declaration has no effect. Otherwise, the program is ill-formed, and the resulting diagnostic message ([intro.compliance]) shall include the text of the string-literal, except that characters not in the basic source character set ([lex.charset]) are not required to appear in the diagnostic message. [ Example:
static_assert(sizeof(long) >= 8, "64-bit code generation required for this library.");
— end example ]
Syntactic components beyond those found in the general form of declaration are added to a function declaration to make a function-definition. An object declaration, however, is also a definition unless it contains the extern specifier and has no initializer ([basic.def]). A definition causes the appropriate amount of storage to be reserved and any appropriate initialization ([dcl.init]) to be done.
Only in function declarations for constructors, destructors, and type conversions can the decl-specifier-seq be omitted.92
7.1 Specifiers [dcl.spec]If a type-name is encountered while parsing a decl-specifier-seq, it is interpreted as part of the decl-specifier-seq if and only if there is no previous type-specifier other than a cv-qualifier in the decl-specifier-seq. The sequence shall be self-consistent as described below. [ Example:
typedef char* Pc; static Pc;
Here, the declaration static Pc is ill-formed because no name was specified for the static variable of type Pc. To get a variable called Pc, a type-specifier (other than const or volatile) has to be present to indicate that the typedef-name Pc is the name being (re)declared, rather than being part of the decl-specifier sequence. For another example,
void f(const Pc); void g(const int Pc);
— end example ]
[ Note: Since signed, unsigned, long, and short by default imply int, a type-name appearing after one of those specifiers is treated as the name being (re)declared. [ Example:
void h(unsigned Pc); void k(unsigned int Pc);
— end example ] — end note ]
7.1.1 Storage class specifiers [dcl.stc]The register specifier shall be applied only to names of variables declared in a block ([stmt.block]) or to function parameters ([dcl.fct.def]). It specifies that the named variable has automatic storage duration ([basic.stc.auto]). A variable declared without a storage-class-specifier at block scope or declared as a function parameter has automatic storage duration by default.
A register specifier is a hint to the implementation that the variable so declared will be heavily used. [ Note: The hint can be ignored and in most implementations it will be ignored if the address of the variable is taken. This use is deprecated (see [depr.register]). — end note ]
The static specifier can be applied only to names of variables and functions and to anonymous unions ([class.union]). There can be no static function declarations within a block, nor any static function parameters. A static specifier used in the declaration of a variable declares the variable to have static storage duration ([basic.stc.static]), unless accompanied by the thread_local specifier, which declares the variable to have thread storage duration ([basic.stc.thread]). A static specifier can be used in declarations of class members; [class.static] describes its effect. For the linkage of a name declared with a static specifier, see [basic.link].
The extern specifier can be applied only to the names of variables and functions. The extern specifier cannot be used in the declaration of class members or function parameters. For the linkage of a name declared with an extern specifier, see [basic.link]. [ Note: The extern keyword can also be used in explicit-instantiations and linkage-specifications, but it is not a storage-class-specifier in such contexts. — end note ]
The linkages implied by successive declarations for a given entity shall agree. That is, within a given scope, each declaration declaring the same variable name or the same overloading of a function name shall imply the same linkage. Each function in a given set of overloaded functions can have a different linkage, however. [ Example:
static char* f(); char* f() { }
char* g(); static char* g() { }
void h();
inline void h();
inline void l();
void l();
inline void m();
extern void m();
static void n();
inline void n();
static int a; int a;
static int b; extern int b;
int c; static int c;
extern int d; static int d;
— end example ]
The name of a declared but undefined class can be used in an extern declaration. Such a declaration can only be used in ways that do not require a complete class type. [ Example:
struct S;
extern S a;
extern S f();
extern void g(S);
void h() {
g(a); f(); }
— end example ]
The mutable specifier can be applied only to names of class data members ([class.mem]) and cannot be applied to names declared const or static, and cannot be applied to reference members. [ Example:
class X {
mutable const int* p; mutable int* const q; };
— end example ]
The mutable specifier on a class data member nullifies a const specifier applied to the containing class object and permits modification of the mutable class member even though the rest of the object is const ([dcl.type.cv]).
7.1.2 Function specifiers [dcl.fct.spec]Function-specifiers can be used only in function declarations.
function-specifier:
inline
virtual
explicit
A function declaration ([dcl.fct], [class.mfct], [class.friend]) with an inline specifier declares an inline function. The inline specifier indicates to the implementation that inline substitution of the function body at the point of call is to be preferred to the usual function call mechanism. An implementation is not required to perform this inline substitution at the point of call; however, even if this inline substitution is omitted, the other rules for inline functions defined by [dcl.fct.spec] shall still be respected.
A function defined within a class definition is an inline function. The inline specifier shall not appear on a block scope function declaration.93 If the inline specifier is used in a friend declaration, that declaration shall be a definition or the function shall have previously been declared inline.
An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case ([basic.def.odr]). [ Note: A call to the inline function may be encountered before its definition appears in the translation unit. — end note ] If the definition of a function appears in a translation unit before its first declaration as inline, the program is ill-formed. If a function with external linkage is declared inline in one translation unit, it shall be declared inline in all translation units in which it appears; no diagnostic is required. An inline function with external linkage shall have the same address in all translation units. A static local variable in an extern inline function always refers to the same object. A string literal in the body of an extern inline function is the same object in different translation units. [ Note: A string literal appearing in a default argument is not in the body of an inline function merely because the expression is used in a function call from that inline function. — end note ] A type defined within the body of an extern inline function is the same type in every translation unit.
The virtual specifier shall be used only in the initial declaration of a non-static class member function; see [class.virtual].
The explicit specifier shall be used only in the declaration of a constructor or conversion function within its class definition; see [class.conv.ctor] and [class.conv.fct].
7.1.3 The typedef specifier [dcl.typedef]A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. It has the same semantics as if it were introduced by the typedef specifier. In particular, it does not define a new type and it shall not appear in the type-id. [ Example:
using handler_t = void (*)(int); extern handler_t ignore; extern void (*ignore)(int); using cell = pair<void*, cell*>;
— end example ]
In a given non-class scope, a typedef specifier can be used to redefine the name of any type declared in that scope to refer to the type to which it already refers. [ Example:
typedef struct s { } s;
typedef int I;
typedef int I;
typedef I I;
— end example ]
In a given class scope, a typedef specifier can be used to redefine any class-name declared in that scope that is not also a typedef-name to refer to the type to which it already refers. [ Example:
struct S {
typedef struct A { } A; typedef struct B B; typedef A A; };
— end example ]
If a typedef specifier is used to redefine in a given scope an entity that can be referenced using an elaborated-type-specifier, the entity can continue to be referenced by an elaborated-type-specifier or as an enumeration or class name in an enumeration or class definition respectively. [ Example:
struct S;
typedef struct S S;
int main() {
struct S* p; }
struct S { };
— end example ]
In a given scope, a typedef specifier shall not be used to redefine the name of any type declared in that scope to refer to a different type. [ Example:
class complex { };
typedef int complex;
— end example ]
Similarly, in a given scope, a class or enumeration shall not be declared with the same name as a typedef-name that is declared in that scope and refers to a type other than the class or enumeration itself. [ Example:
typedef int complex;
class complex { /* ... */ };
— end example ]
If the typedef declaration defines an unnamed class (or enum), the first typedef-name declared by the declaration to be that class type (or enum type) is used to denote the class type (or enum type) for linkage purposes only ([basic.link]). [ Example:
typedef struct { } *ps, S;
— end example ]
7.1.5 The constexpr specifier [dcl.constexpr]The constexpr specifier shall be applied only to the definition of a variable or variable template, the declaration of a function or function template, or the declaration of a static data member of a literal type ([basic.types]). If any declaration of a function, function template, or variable template has a constexpr specifier, then all its declarations shall contain the constexpr specifier. [ Note: An explicit specialization can differ from the template declaration with respect to the constexpr specifier. — end note ] [ Note: Function parameters cannot be declared constexpr. — end note ] [ Example:
constexpr void square(int &x); constexpr int bufsz = 1024; constexpr struct pixel { int x;
int y;
constexpr pixel(int); };
constexpr pixel::pixel(int a)
: x(a), y(x) { square(x); }
constexpr pixel small(2);
constexpr void square(int &x) { x *= x;
}
constexpr pixel large(4); int next(constexpr int x) { return x + 1;
}
extern constexpr int memsz;
— end example ]
A constexpr specifier used in the declaration of a function that is not a constructor declares that function to be a constexpr function. Similarly, a constexpr specifier used in a constructor declaration declares that constructor to be a constexpr constructor. constexpr functions and constexpr constructors are implicitly inline ([dcl.fct.spec]).
The definition of a constexpr function shall satisfy the following constraints:
it shall not be virtual ([class.virtual]);
its return type shall be a literal type;
each of its parameter types shall be a literal type;
its function-body shall be = delete, = default, or a compound-statement that does not contain
an asm-definition,
a goto statement,
a try-block, or
a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.
[ Example:
constexpr int square(int x)
{ return x * x; } constexpr long long_max()
{ return 2147483647; } constexpr int abs(int x) {
if (x < 0)
x = -x;
return x; }
constexpr int first(int n) {
static int value = n; return value;
}
constexpr int uninit() {
int a; return a;
}
constexpr int prev(int x)
{ return --x; } constexpr int g(int x, int n) { int r = 1;
while (--n > 0) r *= x;
return r;
}
— end example ]
The definition of a constexpr constructor shall satisfy the following constraints:
the class shall not have any virtual base classes;
each of the parameter types shall be a literal type;
its function-body shall not be a function-try-block;
In addition, either its function-body shall be = delete, or it shall satisfy the following constraints:
either its function-body shall be = default, or the compound-statement of its function-body shall satisfy the constraints for a function-body of a constexpr function;
every non-variant non-static data member and base class sub-object shall be initialized ([class.base.init]);
if the class is a union having variant members ([class.union]), exactly one of them shall be initialized;
if the class is a union-like class, but is not a union, for each of its anonymous union members having variant members, exactly one of them shall be initialized;
for a non-delegating constructor, every constructor selected to initialize non-static data members and base class sub-objects shall be a constexpr constructor;
for a delegating constructor, the target constructor shall be a constexpr constructor.
[ Example:
struct Length {
constexpr explicit Length(int i = 0) : val(i) { }
private:
int val;
};
— end example ]
For a non-template, non-defaulted constexpr function or a non-template, non-defaulted, non-inheriting constexpr constructor, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression ([expr.const]), the program is ill-formed; no diagnostic required. [ Example:
constexpr int f(bool b)
{ return b ? throw 0 : 0; } constexpr int f() { return f(true); }
struct B {
constexpr B(int x) : i(0) { } int i;
};
int global;
struct D : B {
constexpr D() : B(global) { } };
— end example ]
If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function or constexpr constructor, that specialization is still a constexpr function or constexpr constructor, even though a call to such a function cannot appear in a constant expression. If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.
A call to a constexpr function produces the same result as a call to an equivalent non-constexpr function in all respects except that a call to a constexpr function can appear in a constant expression.
The constexpr specifier has no effect on the type of a constexpr function or a constexpr constructor. [ Example:
constexpr int bar(int x, int y) { return x + y + x*y; }
int bar(int x, int y) { return x * 2 + 3 * y; }
— end example ]
A constexpr specifier used in an object declaration declares the object as const. Such an object shall have literal type and shall be initialized. If it is initialized by a constructor call, that call shall be a constant expression ([expr.const]). Otherwise, or if a constexpr specifier is used in a reference declaration, every full-expression that appears in its initializer shall be a constant expression. [ Note: Each implicit conversion used in converting the initializer expressions and each constructor call used for the initialization is part of such a full-expression. — end note ] [ Example:
struct pixel {
int x, y;
};
constexpr pixel ur = { 1294, 1024 };constexpr pixel origin;
— end example ]
7.1.6 Type specifiers [dcl.type]As a general rule, at most one type-specifier is allowed in the complete decl-specifier-seq of a declaration or in a type-specifier-seq or trailing-type-specifier-seq. The only exceptions to this rule are the following:
const can be combined with any type specifier except itself.
volatile can be combined with any type specifier except itself.
signed or unsigned can be combined with char, long, short, or int.
short or long can be combined with int.
long can be combined with double.
long can be combined with long.
[ Note: Declaring a variable const can affect its linkage ([dcl.stc]) and its usability in constant expressions ([expr.const]). As described in [dcl.init], the definition of an object or subobject of const-qualified type must specify an initializer or be subject to default-initialization. — end note ]
A pointer or reference to a cv-qualified type need not actually point or refer to a cv-qualified object, but it is treated as if it does; a const-qualified access path cannot be used to modify an object even if the object referenced is a non-const object and can be modified through some other access path. [ Note: Cv-qualifiers are supported by the type system so that they cannot be subverted without casting ([expr.const.cast]). — end note ]
Except that any class member declared mutable ([dcl.stc]) can be modified, any attempt to modify a const object during its lifetime ([basic.life]) results in undefined behavior. [ Example:
const int ci = 3; ci = 4; int i = 2; const int* cip; cip = &i; *cip = 4; int* ip; ip = const_cast<int*>(cip); *ip = 4; const int* ciq = new const int (3); int* iq = const_cast<int*>(ciq); *iq = 4;
For another example
struct X {
mutable int i;
int j;
};
struct Y {
X x;
Y();
};
const Y y;
y.x.i++; y.x.j++; Y* p = const_cast<Y*>(&y); p->x.i = 99; p->x.j = 99;
— end example ]
What constitutes an access to an object that has volatile-qualified type is implementation-defined. If an attempt is made to refer to an object defined with a volatile-qualified type through the use of a glvalue with a non-volatile-qualified type, the program behavior is undefined.
[ Note: volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation. Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the object. See [intro.execution] for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C. — end note ]
7.1.6.2 Simple type specifiers [dcl.type.simple]The auto specifier is a placeholder for a type to be deduced ([dcl.spec.auto]). The other simple-type-specifiers specify either a previously-declared type, a type determined from an expression, or one of the fundamental types ([basic.fundamental]). Table [tab:simple.type.specifiers] summarizes the valid combinations of simple-type-specifiers and the types they specify.
Table
10—
simple-type-specifiersand the types they specify
When multiple simple-type-specifiers are allowed, they can be freely intermixed with other decl-specifiers in any order. [ Note: It is implementation-defined whether objects of char type are represented as signed or unsigned quantities. The signed specifier forces char objects to be signed; it is redundant in other contexts. — end note ]
For an expression e, the type denoted by decltype(e) is defined as follows:
if e is an unparenthesized id-expression or an unparenthesized class member access ([expr.ref]), decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;
otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e;
otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;
otherwise, decltype(e) is the type of e.
The operand of the decltype specifier is an unevaluated operand (Clause [expr]).
[ Example:
const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = 0; decltype(i) x2; decltype(a->x) x3; decltype((a->x)) x4 = x3;
— end example ] [ Note: The rules for determining types involving decltype(auto) are specified in [dcl.spec.auto]. — end note ]
[ Note: in the case where the operand of a decltype-specifier is a function call and the return type of the function is a class type, a special rule ([expr.call]) ensures that the return type is not required to be complete (as it would be if the call appeared in a sub-expression or outside of a decltype-specifier). In this context, the common purpose of writing the expression is merely to refer to its type. In that sense, a decltype-specifier is analogous to a use of a typedef-name, so the usual reasons for requiring a complete type do not apply. In particular, it is not necessary to allocate storage for a temporary object or to enforce the semantic constraints associated with invoking the type's destructor. [ Example:
template<class T> struct A { ~A() = delete; };
template<class T> auto h()
-> A<T>;
template<class T> auto i(T) -> T;
template<class T> auto f(T) -> decltype(i(h<T>())); template<class T> auto f(T) -> void;
auto g() -> void {
f(42); }
template<class T> auto q(T)
-> decltype((h<T>())); void r() {
q(42); }
— end example ] — end note ]
7.1.6.4 auto specifier [dcl.spec.auto]The auto and decltype(auto) type-specifiers designate a placeholder type that will be replaced later, either by deduction from an initializer or by explicit specification with a trailing-return-type. The auto type-specifier is also used to signify that a lambda is a generic lambda.
A program that uses auto or decltype(auto) in a context not explicitly allowed in this section is ill-formed.
When a variable declared using a placeholder type is initialized, or a return statement occurs in a function declared with a return type that contains a placeholder type, the deduced return type or variable type is determined from the type of its initializer. In the case of a return with no operand, the initializer is considered to be void(). Let T be the declared type of the variable or return type of the function. If the placeholder is the auto type-specifier, the deduced type is determined using the rules for template argument deduction. If the deduction is for a return statement and the initializer is a braced-init-list ([dcl.init.list]), the program is ill-formed. Otherwise, obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initializer is a braced-init-list, with std::initializer_list<U>. Deduce a value for U using the rules of template argument deduction from a function call ([temp.deduct.call]), where P is a function template parameter type and the initializer is the corresponding argument. If the deduction fails, the declaration is ill-formed. Otherwise, the type deduced for the variable or return type is obtained by substituting the deduced U into P. [ Example:
auto x1 = { 1, 2 }; auto x2 = { 1, 2.0 };
— end example ]
[ Example:
const auto &i = expr;
The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:
template <class U> void f(const U& u);
— end example ]
If the placeholder is the decltype(auto) type-specifier, the declared type of the variable or return type of the function shall be the placeholder alone. The type deduced for the variable or return type is determined as described in [dcl.type.simple], as though the initializer had been the operand of the decltype. [ Example:
int i;
int&& f();
auto x3a = i; decltype(auto) x3d = i; auto x4a = (i); decltype(auto) x4d = (i); auto x5a = f(); decltype(auto) x5d = f(); auto x6a = { 1, 2 }; decltype(auto) x6d = { 1, 2 }; auto *x7a = &i; decltype(auto)*x7d = &i;
— end example ]
If the init-declarator-list contains more than one init-declarator, they shall all form declarations of variables. The type of each declared variable is determined as described above, and if the type that replaces the placeholder type is not the same in each deduction, the program is ill-formed.
[ Example:
auto x = 5, *y = &x; auto a = 5, b = { 1, 2 };
If a function with a declared return type that contains a placeholder type has multiple return statements, the return type is deduced for each return statement. If the type deduced is not the same in each deduction, the program is ill-formed.
If a function with a declared return type that uses a placeholder type has no return statements, the return type is deduced as though from a return statement with no operand at the closing brace of the function body. [ Example:
auto f() { } auto* g() { }
— end example ]
If the type of an entity with an undeduced placeholder type is needed to determine the type of an expression, the program is ill-formed. Once a return statement has been seen in a function, however, the return type deduced from that statement can be used in the rest of the function, including in other return statements. [ Example:
auto n = n; auto f();
void g() { &f; } auto sum(int i) {
if (i == 1)
return i; else
return sum(i-1)+i; }
— end example ]
Return type deduction for a function template with a placeholder in its declared type occurs when the definition is instantiated even if the function body contains a return statement with a non-type-dependent operand. [ Note: Therefore, any use of a specialization of the function template will cause an implicit instantiation. Any errors that arise from this instantiation are not in the immediate context of the function type and can result in the program being ill-formed. — end note ] [ Example:
template <class T> auto f(T t) { return t; } typedef decltype(f(1)) fint_t; template<class T> auto f(T* t) { return *t; }
void g() { int (*p)(int*) = &f; }
— end example ]
Redeclarations or specializations of a function or function template with a declared return type that uses a placeholder type shall also use that placeholder, not a deduced type. [ Example:
auto f();
auto f() { return 42; } auto f(); int f(); decltype(auto) f();
template <typename T> auto g(T t) { return t; } template auto g(int); template char g(char); template<> auto g(double);
template <class T> T g(T t) { return t; } template char g(char); template auto g(float);
void h() { return g(42); }
template <typename T> struct A {
friend T frf(T);
};
auto frf(int i) { return i; }
— end example ]
A function declared with a return type that uses a placeholder type shall not be virtual ([class.virtual]).
An explicit instantiation declaration ([temp.explicit]) does not cause the instantiation of an entity declared using a placeholder type, but it also does not prevent that entity from being instantiated as needed to determine its type. [ Example:
template <typename T> auto f(T t) { return t; }
extern template auto f(int); int (*p)(int) = f;
— end example ]
7.2 Enumeration declarations [dcl.enum]The enumeration type declared with an enum-key of only enum is an unscoped enumeration, and its enumerators are unscoped enumerators. The enum-keys enum class and enum struct are semantically equivalent; an enumeration type declared with one of these is a scoped enumeration, and its enumerators are scoped enumerators. The optional identifier shall not be omitted in the declaration of a scoped enumeration. The type-specifier-seq of an enum-base shall name an integral type; any cv-qualification is ignored. An opaque-enum-declaration declaring an unscoped enumeration shall not omit the enum-base. The identifiers in an enumerator-list are declared as constants, and can appear wherever constants are required. An enumerator-definition with = gives the associated enumerator the value indicated by the constant-expression. If the first enumerator has no initializer, the value of the corresponding constant is zero. An enumerator-definition without an initializer gives the enumerator the value obtained by increasing the value of the previous enumerator by one.
[ Example:
enum { a, b, c=0 };
enum { d, e, f=e+2 };
defines a, c, and d to be zero, b and e to be 1, and f to be 3. — end example ]
An opaque-enum-declaration is either a redeclaration of an enumeration in the current scope or a declaration of a new enumeration. [ Note: An enumeration declared by an opaque-enum-declaration has fixed underlying type and is a complete type. The list of enumerators can be provided in a later redeclaration with an enum-specifier. — end note ] A scoped enumeration shall not be later redeclared as unscoped or with a different underlying type. An unscoped enumeration shall not be later redeclared as scoped and each redeclaration shall include an enum-base specifying the same underlying type as in the original declaration.
Each enumeration defines a type that is different from all other types. Each enumeration also has an underlying type. The underlying type can be explicitly specified using an enum-base. For a scoped enumeration type, the underlying type is int if it is not explicitly specified. In both of these cases, the underlying type is said to be fixed. Following the closing brace of an enum-specifier, each enumerator has the type of its enumeration. If the underlying type is fixed, the type of each enumerator prior to the closing brace is the underlying type and the constant-expression in the enumerator-definition shall be a converted constant expression of the underlying type ([expr.const]). If the underlying type is not fixed, the type of each enumerator prior to the closing brace is determined as follows:
If an initializer is specified for an enumerator, the constant-expression shall be an integral constant expression ([expr.const]). If the expression has unscoped enumeration type, the enumerator has the underlying type of that enumeration type, otherwise it has the same type as the expression.
If no initializer is specified for the first enumerator, its type is an unspecified signed integral type.
Otherwise the type of the enumerator is the same as that of the preceding enumerator unless the incremented value is not representable in that type, in which case the type is an unspecified integral type sufficient to contain the incremented value. If no such type exists, the program is ill-formed.
An enumeration whose underlying type is fixed is an incomplete type from its point of declaration ([basic.scope.pdecl]) to immediately after its enum-base (if any), at which point it becomes a complete type. An enumeration whose underlying type is not fixed is an incomplete type from its point of declaration to immediately after the closing } of its enum-specifier, at which point it becomes a complete type.
For an enumeration whose underlying type is not fixed, the underlying type is an integral type that can represent all the enumerator values defined in the enumeration. If no integral type can represent all the enumerator values, the enumeration is ill-formed. It is implementation-defined which integral type is used as the underlying type except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int. If the enumerator-list is empty, the underlying type is as if the enumeration had a single enumerator with value 0.
For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type. Otherwise, for an enumeration where emin is the smallest enumerator and emax is the largest, the values of the enumeration are the values in the range bmin to bmax, defined as follows: Let K be 1 for a two's complement representation and 0 for a one's complement or sign-magnitude representation. bmax is the smallest value greater than or equal to max(|emin| - K, |emax|) and equal to 2M-1, where M is a non-negative integer. bmin is zero if emin is non-negative and -(bmax+K) otherwise. The size of the smallest bit-field large enough to hold all the values of the enumeration type is max(M,1) if bmin is zero and M+1 otherwise. It is possible to define an enumeration that has values not defined by any of its enumerators. If the enumerator-list is empty, the values of the enumeration are as if the enumeration had a single enumerator with value 0.95
The value of an enumerator or an object of an unscoped enumeration type is converted to an integer by integral promotion ([conv.prom]). [ Example:
enum color { red, yellow, green=20, blue };
color col = red;
color* cp = &col;
if (*cp == blue)
makes color a type describing various colors, and then declares col as an object of that type, and cp as a pointer to an object of that type. The possible values of an object of type color are red, yellow, green, blue; these values can be converted to the integral values 0, 1, 20, and 21. Since enumerations are distinct types, objects of type color can be assigned only values of type color.
color c = 1; int i = yellow;
Note that this implicit enum to int conversion is not provided for a scoped enumeration:
enum class Col { red, yellow, green };
int x = Col::red; Col y = Col::red;
if (y) { }
— end example ]
Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier. Each scoped enumerator is declared in the scope of the enumeration. These names obey the scope rules defined for all names in ([basic.scope]) and ([basic.lookup]).[ Example:
enum direction { left='l', right='r' };
void g() {
direction d; d = left; d = direction::right; }
enum class altitude { high='h', low='l' };
void h() {
altitude a; a = high; a = altitude::low; }
— end example ] An enumerator declared in class scope can be referred to using the class member access operators (::, . (dot) and -> (arrow)), see [expr.ref]. [ Example:
struct X {
enum direction { left='l', right='r' };
int f(int i) { return i==left ? 0 : i==right ? 1 : 2; }
};
void g(X* p) {
direction d; int i;
i = p->f(left); i = p->f(X::right); i = p->f(p->left); }
— end example ]
7.3 Namespaces [basic.namespace]A namespace is an optionally-named declarative region. The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units.
7.3.1 Namespace definition [namespace.def]The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as specified in [namespace.memdef]). Such a redeclaration has the same enclosing namespaces as the original declaration. [ Example:
namespace Q {
namespace V {
void f(); class C { void m(); };
}
void V::f() { extern void h(); }
void V::C::m() { }
}
— end example ]
Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup ([basic.lookup.argdep]) whenever one of them is, and a using-directive ([namespace.udir]) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace ([namespace.unnamed]). Furthermore, each member of the inline namespace can subsequently be explicitly instantiated ([temp.explicit]) or explicitly specialized ([temp.expl.spec]) as though it were a member of the enclosing namespace. Finally, looking up a name in the enclosing namespace via explicit qualification ([namespace.qual]) will include members of the inline namespace brought in by the using-directive even if there are declarations of that name in the enclosing namespace.
These properties are transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O, then the members of O can be used as though they were members of M or N. The inline namespace set of N is the transitive closure of all inline namespaces in N. The enclosing namespace set of O is the set of namespaces consisting of the innermost non-inline namespace enclosing an inline namespace O, together with any intervening inline namespaces.
7.3.1.1 Unnamed namespaces [namespace.unnamed]An unnamed-namespace-definition behaves as if it were replaced by
inlineopt namespace unique { /* empty body */ }
using namespace unique ;
namespace unique { namespace-body }
where inline appears if and only if it appears in the unnamed-namespace-definition, all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the entire program.96 [ Example:
namespace { int i; } void f() { i++; }
namespace A {
namespace {
int i; int j; }
void g() { i++; } }
using namespace A;
void h() {
i++; A::i++; j++; }
— end example ]
7.3.1.2 Namespace member definitions [namespace.memdef]Members (including explicit specializations of templates ([temp.expl.spec])) of a namespace can be defined within that namespace. [ Example:
namespace X {
void f() { }
}
— end example ]
Members of a named namespace can also be defined outside that namespace by explicit qualification ([namespace.qual]) of the name being defined, provided that the entity being defined was already declared in the namespace and the definition appears after the point of declaration in a namespace that encloses the declaration's namespace. [ Example:
namespace Q {
namespace V {
void f();
}
void V::f() { } void V::g() { } namespace V {
void g();
}
}
namespace R {
void Q::V::g() { } }
— end example ]
Every name first declared in a namespace is a member of that namespace. If a friend declaration in a non-local class first declares a class, function, class template or function template97 the friend is a member of the innermost enclosing namespace. The friend declaration does not by itself make the name visible to unqualified lookup ([basic.lookup.unqual]) or qualified lookup ([basic.lookup.qual]). [ Note: The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship). — end note ] If a friend function or function template is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments ([basic.lookup.argdep]). If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace. [ Note: The other forms of friend declarations cannot declare a new member of the innermost enclosing namespace and thus follow the usual lookup rules. — end note ] [ Example:
void h(int);
template <class T> void f2(T);
namespace A {
class X {
friend void f(X); class Y {
friend void g(); friend void h(int); friend void f2<>(int); };
};
X x;
void g() { f(x); } void f(X) { /* ... */} void h(int) { /* ... */ } }
using A::x;
void h() {
A::f(x);
A::X::f(x); A::X::Y::g(); }
— end example ]
7.3.2 Namespace alias [namespace.alias]In a declarative region, a namespace-alias-definition can be used to redefine a namespace-alias declared in that declarative region to refer only to the namespace to which it already refers. [ Example: the following declarations are well-formed:
namespace Company_with_very_long_name { }
namespace CWVLN = Company_with_very_long_name;
namespace CWVLN = Company_with_very_long_name; namespace CWVLN = CWVLN;
— end example ]
A namespace-name or namespace-alias shall not be declared as the name of any other entity in the same declarative region. A namespace-name defined at global scope shall not be declared as the name of any other entity in any global scope of the program. No diagnostic is required for a violation of this rule by declarations in different translation units.
7.3.3 The using declaration [namespace.udecl]Every using-declaration is a declaration and a member-declaration and so can be used in a class definition. [ Example:
struct B {
void f(char);
void g(char);
enum E { e };
union { int x; };
};
struct D : B {
using B::f;
void f(int) { f('c'); } void g(int) { g('c'); } };
— end example ]
[ Note: Since destructors do not have names, a using-declaration cannot refer to a destructor for a base class. Since specializations of member templates for conversion functions are not found by name lookup, they are not considered when a using-declaration specifies a conversion function ([temp.mem]). — end note ] If an assignment operator brought from a base class into a derived class scope has the signature of a copy/move assignment operator for the derived class ([class.copy]), the using-declaration does not by itself suppress the implicit declaration of the derived class assignment operator; the copy/move assignment operator from the base class is hidden or overridden by the implicitly-declared copy/move assignment operator of the derived class, as described below.
A using-declaration shall not name a template-id. [ Example:
struct A {
template <class T> void f(T);
template <class T> struct X { };
};
struct B : A {
using A::f<double>; using A::X<int>; };
— end example ]
Members declared by a using-declaration can be referred to by explicit qualification just like other member names ([namespace.qual]). In a using-declaration, a prefix :: refers to the global namespace. [ Example:
void f();
namespace A {
void g();
}
namespace X {
using ::f; using A::g; }
void h(){
X::f(); X::g(); }
— end example ]
A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed. [ Example:
namespace A {
int i;
}
namespace A1 {
using A::i;
using A::i; }
void f() {
using A::i;
using A::i; }
struct B {
int i;
};
struct X : B {
using B::i;
using B::i; };
— end example ]
Members added to the namespace after the using-declaration are not considered when a use of the name is made. [ Note: Thus, additional overloads added after the using-declaration are ignored, but default function arguments ([dcl.fct.default]), default template arguments ([temp.param]), and template specializations ([temp.class.spec], [temp.expl.spec]) are considered. — end note ] [ Example:
namespace A {
void f(int);
}
using A::f; namespace A {
void f(char);
}
void foo() {
f('a'); }
void bar() {
using A::f; f('a'); }
— end example ]
[ Note: Partial specializations of class templates are found by looking up the primary class template and then considering all partial specializations of that template. If a using-declaration names a class template, partial specializations introduced after the using-declaration are effectively visible because the primary template is visible ([temp.class.spec]). — end note ]
Since a using-declaration is a declaration, the restrictions on declarations of the same name in the same declarative region ([basic.scope]) also apply to using-declarations. [ Example:
namespace A {
int x;
}
namespace B {
int i;
struct g { };
struct x { };
void f(int);
void f(double);
void g(char); }
void func() {
int i;
using B::i; void f(char);
using B::f; f(3.5); using B::g;
g('a'); struct g g1; using B::x;
using A::x; x = 99; struct x x1; }
— end example ]
If a function declaration in namespace scope or block scope has the same name and the same parameter-type-list ([dcl.fct]) as a function introduced by a using-declaration, and the declarations do not declare the same function, the program is ill-formed. If a function template declaration in namespace scope has the same name, parameter-type-list, return type, and template parameter list as a function template introduced by a using-declaration, the program is ill-formed. [ Note: Two using-declarations may introduce functions with the same name and the same parameter-type-list. If, for a call to an unqualified function name, function overload resolution selects the functions introduced by such using-declarations, the function call is ill-formed. [ Example:
namespace B {
void f(int);
void f(double);
}
namespace C {
void f(int);
void f(double);
void f(char);
}
void h() {
using B::f; using C::f; f('h'); f(1); void f(int); }
— end example ] — end note ]
When a using-declaration brings names from a base class into a derived class scope, member functions and member function templates in the derived class override and/or hide member functions and member function templates with the same name, parameter-type-list ([dcl.fct]), cv-qualification, and ref-qualifier (if any) in a base class (rather than conflicting). [ Note: For using-declarations that name a constructor, see [class.inhctor]. — end note ] [ Example:
struct B {
virtual void f(int);
virtual void f(char);
void g(int);
void h(int);
};
struct D : B {
using B::f;
void f(int);
using B::g;
void g(char);
using B::h;
void h(int); };
void k(D* p){
p->f(1); p->f('a'); p->g(1); p->g('a'); }
— end example ]
For the purpose of overload resolution, the functions which are introduced by a using-declaration into a derived class will be treated as though they were members of the derived class. In particular, the implicit this parameter shall be treated as if it were a pointer to the derived class rather than to the base class. This has no effect on the type of the function, and in all other respects the function remains a member of the base class.
The access rules for inheriting constructors are specified in [class.inhctor]; otherwise all instances of the name mentioned in a using-declaration shall be accessible. In particular, if a derived class uses a using-declaration to access a member of a base class, the member name shall be accessible. If the name is that of an overloaded member function, then all functions named shall be accessible. The base class members mentioned by a using-declaration shall be visible in the scope of at least one of the direct base classes of the class where the using-declaration is specified. [ Note: Because a using-declaration designates a base class member (and not a member subobject or a member function of a base class subobject), a using-declaration cannot be used to resolve inherited member ambiguities. For example,
struct A { int x(); };
struct B : A { };
struct C : A {
using A::x;
int x(int);
};
struct D : B, C {
using C::x;
int x(double);
};
int f(D* d) {
return d->x(); }
— end note ]
The alias created by the using-declaration has the usual accessibility for a member-declaration. [ Note: A using-declaration that names a constructor does not create aliases; see [class.inhctor] for the pertinent accessibility rules. — end note ] [ Example:
class A {
private:
void f(char);
public:
void f(int);
protected:
void g();
};
class B : public A {
using A::f; public:
using A::g; };
— end example ]
7.3.4 Using directive [namespace.udir]using-directive:
attribute-specifier-seqopt using namespace nested-name-specifieropt namespace-name ;
A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive. During unqualified name lookup ([basic.lookup.unqual]), the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace. [ Note: In this context, “contains” means “contains directly or indirectly”. — end note ]
A using-directive does not add any members to the declarative region in which it appears. [ Example:
namespace A {
int i;
namespace B {
namespace C {
int i;
}
using namespace A::B::C;
void f1() {
i = 5; }
}
namespace D {
using namespace B;
using namespace C;
void f2() {
i = 5; }
}
void f3() {
i = 5; }
}
void f4() {
i = 5; }
— end example ]
For unqualified lookup ([basic.lookup.unqual]), the using-directive is transitive: if a scope contains a using-directive that nominates a second namespace that itself contains using-directives, the effect is as if the using-directives from the second namespace also appeared in the first. [ Note: For qualified lookup, see [namespace.qual]. — end note ] [ Example:
namespace M {
int i;
}
namespace N {
int i;
using namespace M;
}
void f() {
using namespace N;
i = 7; }
For another example,
namespace A {
int i;
}
namespace B {
int i;
int j;
namespace C {
namespace D {
using namespace A;
int j;
int k;
int a = i; }
using namespace D;
int k = 89; int l = k; int m = i; int n = j; }
}
— end example ]
If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed. [ Note: In particular, the name of a variable, function or enumerator does not hide the name of a class or enumeration declared in a different namespace. For example,
namespace A {
class X { };
extern "C" int g();
extern "C++" int h();
}
namespace B {
void X(int);
extern "C" int g();
extern "C++" int h(int);
}
using namespace A;
using namespace B;
void f() {
X(1); g(); h(); }
— end note ]
During overload resolution, all functions from the transitive search are considered for argument matching. The set of declarations found by the transitive search is unordered. [ Note: In particular, the order in which namespaces were considered and the relationships among the namespaces implied by the using-directives do not cause preference to be given to any of the declarations found by the search. — end note ] An ambiguity exists if the best match finds two functions with the same signature, even if one is in a namespace reachable through using-directives in the namespace of the other.98 [ Example:
namespace D {
int d1;
void f(char);
}
using namespace D;
int d1;
namespace E {
int e;
void f(int);
}
namespace D { int d2;
using namespace E;
void f(int);
}
void f() {
d1++; ::d1++; D::d1++; d2++; e++; f(1); f('a'); }
7.4 The asm declaration [dcl.asm]
An asm declaration has the form
asm-definition:
asm ( string-literal ) ;
The asm declaration is conditionally-supported; its meaning is implementation-defined. [ Note: Typically it is used to pass information through the implementation to an assembler. — end note ]
7.5 Linkage specifications [dcl.link]All function types, function names with external linkage, and variable names with external linkage have a language linkage. [ Note: Some of the properties associated with an entity with language linkage are specific to each implementation and are not described here. For example, a particular language linkage may be associated with a particular form of representing names of objects and functions with external linkage, or with a particular calling convention, etc. — end note ] The default language linkage of all function types, function names, and variable names is C++ language linkage. Two function types with different language linkages are distinct types even if they are otherwise identical.
Every implementation shall provide for linkage to functions written in the C programming language, "C", and linkage to C++ functions, "C++". [ Example:
complex sqrt(complex); extern "C" {
double sqrt(double); }
— end example ]
Linkage specifications nest. When linkage specifications nest, the innermost one determines the language linkage. A linkage specification does not establish a scope. A linkage-specification shall occur only in namespace scope ([basic.scope]). In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification. [ Example:
extern "C" void f1(void(*pf)(int));
extern "C" typedef void FUNC();
FUNC f2; extern "C" FUNC f3; void (*pf2)(FUNC*); extern "C" {
static void f4(); }
extern "C" void f5() {
extern void f4(); }
extern void f4();
void f6() {
extern void f4(); }
— end example ] A C language linkage is ignored in determining the language linkage of the names of class members and the function type of class member functions. [ Example:
extern "C" typedef void FUNC_c();
class C {
void mf1(FUNC_c*); FUNC_c mf2; static FUNC_c* q; };
extern "C" {
class X {
void mf(); void mf2(void(*)()); };
}
— end example ]
If two declarations declare functions with the same name and parameter-type-list ([dcl.fct]) to be members of the same namespace or declare objects with the same name to be members of the same namespace and the declarations give the names different language linkages, the program is ill-formed; no diagnostic is required if the declarations appear in different translation units. Except for functions with C++ linkage, a function declaration without a linkage specification shall not precede the first linkage specification for that function. A function can be declared without a linkage specification after an explicit linkage specification has been seen; the linkage explicitly specified in the earlier declaration is not affected by such a function declaration.
At most one function with a particular name can have C language linkage. Two declarations for a function with C language linkage with the same function name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same function. Two declarations for a variable with C language linkage with the same name (ignoring the namespace names that qualify it) that appear in different namespace scopes refer to the same variable. An entity with C language linkage shall not be declared with the same name as an entity in global scope, unless both declarations denote the same entity; no diagnostic is required if the declarations appear in different translation units. A variable with C language linkage shall not be declared with the same name as a function with C language linkage (ignoring the namespace names that qualify the respective names); no diagnostic is required if the declarations appear in different translation units. [ Note: Only one definition for an entity with a given name with C language linkage may appear in the program (see [basic.def.odr]); this implies that such an entity must not be defined in more than one namespace scope. — end note ] [ Example:
int x;
namespace A {
extern "C" int f();
extern "C" int g() { return 1; }
extern "C" int h();
extern "C" int x(); }
namespace B {
extern "C" int f(); extern "C" int g() { return 1; } }
int A::f() { return 98; } extern "C" int h() { return 97; }
— end example ]
A declaration directly contained in a linkage-specification is treated as if it contains the extern specifier ([dcl.stc]) for the purpose of determining the linkage of the declared name and whether it is a definition. Such a declaration shall not specify a storage class. [ Example:
extern "C" double f();
static double f(); extern "C" int i; extern "C" {
int i; }
extern "C" static void g();
— end example ]
[ Note: Because the language linkage is part of a function type, when indirecting through a pointer to C function, the function to which the resulting lvalue refers is considered a C function. — end note ]
Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved.
7.6 Attributes [dcl.attr] 7.6.1 Attribute syntax and semantics [dcl.attr.grammar][ Note: For each individual attribute, the form of the balanced-token-seq will be specified. — end note ]
For an attribute-token not specified in this International Standard, the behavior is implementation-defined.
Two consecutive left square bracket tokens shall appear only when introducing an attribute-specifier. [ Note: If two consecutive left square brackets appear where an attribute-specifier is not allowed, the program is ill-formed even if the brackets match an alternative grammar production. — end note ] [ Example:
int p[10];
void f() {
int x = 42, y[5];
int(p[[x] { return x; }()]); y[[] { return 2; }()] = 2; }
— end example ]
7.6.2 Alignment specifier [dcl.align]When the alignment-specifier is of the form alignas( constant-expression ):
the constant-expression shall be an integral constant expression
if the constant expression evaluates to a fundamental alignment, the alignment requirement of the declared entity shall be the specified fundamental alignment
if the constant expression evaluates to an extended alignment and the implementation supports that alignment in the context of the declaration, the alignment of the declared entity shall be that alignment
if the constant expression evaluates to an extended alignment and the implementation does not support that alignment in the context of the declaration, the program is ill-formed
if the constant expression evaluates to zero, the alignment specifier shall have no effect
otherwise, the program is ill-formed.
When multiple alignment-specifiers are specified for an entity, the alignment requirement shall be set to the strictest specified alignment.
The combined effect of all alignment-specifiers in a declaration shall not specify an alignment that is less strict than the alignment that would be required for the entity being declared if all alignment-specifiers were omitted (including those in other declarations).
If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.
[ Example:
struct S { int x; } s, p = &s;
struct alignas(16) S; extern S* p;
— end example ]
[ Example: An aligned buffer with an alignment requirement of A and holding N elements of type T other than char, signed char, or unsigned char can be declared as:
alignas(T) alignas(A) T buffer[N];
Specifying alignas(T) ensures that the final requested alignment will not be weaker than alignof(T), and therefore the program will not be ill-formed. — end example ]
[ Example:
alignas(double) void f(); alignas(double) unsigned char c[sizeof(double)]; extern unsigned char c[sizeof(double)]; alignas(float) extern unsigned char c[sizeof(double)];
— end example ]
7.6.3 Noreturn attribute [dcl.attr.noreturn]The attribute-token noreturn specifies that a function does not return. It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present. The attribute may be applied to the declarator-id in a function declaration. The first declaration of a function shall specify the noreturn attribute if any declaration of that function specifies the noreturn attribute. If a function is declared with the noreturn attribute in one translation unit and the same function is declared without the noreturn attribute in another translation unit, the program is ill-formed; no diagnostic required.
If a function f is called where f was previously declared with the noreturn attribute and f eventually returns, the behavior is undefined. [ Note: The function may terminate by throwing an exception. — end note ] [ Note: Implementations are encouraged to issue a warning if a function marked [[noreturn]] might return. — end note ]
[ Example:
[[ noreturn ]] void f() {
throw "error"; }
[[ noreturn ]] void q(int i) { if (i > 0)
throw "positive";
}
— end example ]
7.6.4 Carries dependency attribute [dcl.attr.depend]The attribute-token carries_dependency specifies dependency propagation into and out of functions. It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present. The attribute may be applied to the declarator-id of a parameter-declaration in a function declaration or lambda, in which case it specifies that the initialization of the parameter carries a dependency to ([intro.multithread]) each lvalue-to-rvalue conversion ([conv.lval]) of that object. The attribute may also be applied to the declarator-id of a function declaration, in which case it specifies that the return value, if any, carries a dependency to the evaluation of the function call expression.
The first declaration of a function shall specify the carries_dependency attribute for its declarator-id if any declaration of the function specifies the carries_dependency attribute. Furthermore, the first declaration of a function shall specify the carries_dependency attribute for a parameter if any declaration of that function specifies the carries_dependency attribute for that parameter. If a function or one of its parameters is declared with the carries_dependency attribute in its first declaration in one translation unit and the same function or one of its parameters is declared without the carries_dependency attribute in its first declaration in another translation unit, the program is ill-formed; no diagnostic required.
[ Note: The carries_dependency attribute does not change the meaning of the program, but may result in generation of more efficient code. — end note ]
[ Example:
struct foo { int* a; int* b; };
std::atomic<struct foo *> foo_head[10];
int foo_array[10][10];
[[carries_dependency]] struct foo* f(int i) {
return foo_head[i].load(memory_order_consume);
}
int g(int* x, int* y [[carries_dependency]]) {
return kill_dependency(foo_array[*x][*y]);
}
[[carries_dependency]] struct foo* f(int i);
int g(int* x, int* y [[carries_dependency]]);
int c = 3;
void h(int i) {
struct foo* p;
p = f(i);
do_something_with(g(&c, p->a));
do_something_with(g(p->a, &c));
}
The carries_dependency attribute on function f means that the return value carries a dependency out of f, so that the implementation need not constrain ordering upon return from f. Implementations of f and its caller may choose to preserve dependencies instead of emitting hardware memory ordering instructions (a.k.a. fences).
Function g's second parameter has a carries_dependency attribute, but its first parameter does not. Therefore, function h's first call to g carries a dependency into g, but its second call does not. The implementation might need to insert a fence prior to the second call to g.
7.6.5 Deprecated attribute [dcl.attr.deprecated]The attribute-token deprecated can be used to mark names and entities whose use is still allowed, but is discouraged for some reason. [ Note: in particular, deprecated is appropriate for names and entities that are deemed obsolescent or unsafe. — end note ] It shall appear at most once in each attribute-list. An attribute-argument-clause may be present and, if present, it shall have the form:
( string-literal )
[ Note: the string-literal in the attribute-argument-clause could be used to explain the rationale for deprecation and/or to suggest a replacing entity. — end note ]
The attribute may be applied to the declaration of a class, a typedef-name, a variable, a non-static data member, a function, an enumeration, or a template specialization.
A name or entity declared without the deprecated attribute can later be re-declared with the attribute and vice-versa. [ Note: Thus, an entity initially declared without the attribute can be marked as deprecated by a subsequent redeclaration. However, after an entity is marked as deprecated, later redeclarations do not un-deprecate the entity. — end note ] Redeclarations using different forms of the attribute (with or without the attribute-argument-clause or with different attribute-argument-clauses) are allowed.
[ Note: Implementations may use the deprecated attribute to produce a diagnostic message in case the program refers to a name or entity other than to declare it, after a declaration that specifies the attribute. The diagnostic message may include the text provided within the attribute-argument-clause of any deprecated attribute applied to the name or entity. — end note ]
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4