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Other operators - cppreference.com

Operator
name Syntax Overloadable Prototype examples (for class T) Inside class definition Outside class definition function call a(a1, a2) Yes R T::operator()(Arg1 &a1, Arg2 &a2, ...); N/A comma a, b Yes T2& T::operator,(T2 &b); T2& operator,(const T &a, T2 &b); conditional operator a ? b : c No N/A N/A

The function call operator provides function semantics for any object.

The conditional operator (colloquially referred to as ternary conditional ) checks the boolean value of the first expression and, depending on the resulting value, evaluates and returns either the second or the third expression.

[edit] Built-in function call operator

Function call expressions have the following form:

function (arg1, arg2, arg3,...) function - an expression function type or function pointer type arg1, arg2, arg3,... - a possibly empty list of arbitrary expressions or brace-enclosed initializer lists(since C++11), except the comma operator is not allowed at the top level to avoid ambiguity

For a call to a non-member function or to a static member function, function can be an lvalue that refers to a function (in which case the function-to-pointer conversion is suppressed), or a prvalue of function pointer type.

The function (or member) name specified by function can be overloaded, overload resolution rules used to decide which overload is to be called.

If function specifies a member function, it may be virtual, in which case the final overrider of that function will be called, using dynamic dispatch at runtime.

Each function parameter is initialized with its corresponding argument after implicit conversion if necessary.

If there is no corresponding argument, the corresponding default argument is used, and if there is none, the program is ill-formed.
If the call is made to a member function, then the this pointer to current object is converted as if by explicit cast to the this pointer expected by the function.
The initialization and destruction of each parameter occurs in the context of the full-expression where the function call appears, which means, for example, that if a constructor or destructor of a parameter throws an exception, the function try blocks of the called function are not considered.

If the function is a variadic function, default argument promotions are applied to all arguments matched by the ellipsis parameter.

It is implementation-defined whether a parameter is destroyed when the function in which it is defined exits or at the end of the enclosing full-expression. Parameters are always destroyed in the reverse order of their construction.

The return type of a function call expression is the return type of the chosen function, decided using static binding (ignoring the virtual keyword), even if the overriding function that is actually called returns a different type. This allows the overriding functions to return pointers or references to classes that are derived from the return type returned by the base function, i.e. C++ supports covariant return types. If function specifies a destructor, the return type is void.

When an object of class type X is passed to or returned from a function, if each copy constructor, move constructor, and destructor of X is either trivial or deleted, and X has at least one non-deleted copy or move constructor, implementations are permitted to create a temporary object to hold the function parameter or result object.

The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the non-deleted trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object).

This allows objects of small class types, such as std::complex or std::span, to be passed to or returned from functions in registers.

(since C++17)

The value category of a function call expression is lvalue if the function returns an lvalue reference or an rvalue reference to function, is an xvalue if the function returns an rvalue reference to object, and is a prvalue otherwise. If the function call expression is a prvalue of object type, it must have complete type except when used as the operand of decltype (or as the right operand of a built-in comma operator that is the operand of decltype)(since C++11).

Function call expression is similar in syntax to value initialization T(), to function-style cast expression T(A1), and to direct initialization of a temporary T(A1, A2, A3, ...), where T is the name of a type.

#include <cstdio>
 
struct S
{
    int f1(double d)
    {
        return printf("%f \n", d); // variable argument function call
    }
 
    int f2()
    {
        return f1(7); // member function call, same as this->f1()
                      // integer argument converted to double
    }
};
 
void f()
{
    puts("function called"); // function call
}
 
int main()
{
    f();    // function call
    S s;
    s.f2(); // member function call
}

Output:

[edit] Built-in comma operator

Comma expressions have the following form:

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded (although if it has class type, it won't be destroyed until the end of the containing full expression), and its side effects are completed before evaluation of the expression E2 begins (note that a user-defined operator, cannot guarantee sequencing)(until C++17).

The type, value, and value category of the result of the comma expression are exactly the type, value, and value category of the second operand, E2. If E2 is a temporary expression(since C++17), the result of the expression is that temporary expression(since C++17). If E2 is a bit-field, the result is a bit-field.

The comma in various comma-separated lists, such as function argument lists (f(a, b, c)) and initializer lists int a[] = {1, 2, 3}, is not the comma operator. If the comma operator needs to be used in such contexts, it has to be parenthesized: f(a, (n++, n + b), c).

Using an unparenthesized comma expression as second (right) argument of a subscript operator is deprecated.

For example, a[b, c] is deprecated and a[(b, c)] is not.

(since C++20)
(until C++23)

An unparenthesized comma expression cannot be second (right) argument of a subscript operator. For example, a[b, c] is either ill-formed or equivalent to a.operator[](b, c).

Parentheses are needed when using a comma expression as the subscript, e.g., a[(b, c)].

(since C++23)

#include <iostream>
 
int main()
{
    // comma is often used to execute more than one expression
    // where the language grammar allows only one expression:
 
    // * in the third component of the for loop
    for (int i = 0, j = 10; i <= j; ++i, --j)
    //            ^list separator      ^comma operator
        std::cout << "i = " << i << " j = " << j << '\n';
 
    // * in a return statement
    // return log("an error!"), -1;
 
    // * in an initializer expression
    // MyClass(const Arg& arg)
    // : member{ throws_if_bad(arg), arg }
 
    // etc.
 
    // comma operators can be chained; the result of the last
    // (rightmost) expression is the result of the whole chain:
    int n = 1;
    int m = (++n, std::cout << "n = " << n << '\n', ++n, 2 * n);
 
    // m is now 6
    std::cout << "m = " << (++m, m) << '\n';
}

Output:

i = 0 j = 10
i = 1 j = 9
i = 2 j = 8
i = 3 j = 7
i = 4 j = 6
i = 5 j = 5
n = 2
m = 7

[edit] Conditional operator

The conditional operator expressions have the form

E1 is evaluated and contextually converted to bool, if the result is true, the result of the conditional expression is the value of E2 ; otherwise the result of the conditional expression is the value of E3 .

The type and value category of the conditional expression E1 ? E2 : E3 are determined as follows:

[edit] Stage 1

If both E2 and E3 are of type void, the result is an rvalue(until C++11)a prvalue(since C++11) of type void.

If exactly one of E2 and E3 is of type void:

If that operand of type void is be a (possibly parenthesized) throw expression, the result has the type and the value category of the other operand^[1]. If the other operand is a bit-field, the result is also a bit-field.
Otherwise, the program is ill-formed.

If neither of E2 and E3 is of type void, proceed to the next stage.

2 + 2 == 4 ? throw 123 : throw 456; // the result is of type âvoidâ
 
2 + 2 != 4 ? "OK" : throw "error";  // the result is of type âconst char[3]â
                                    // even if an exception is always thrown

[edit] Stage 2

If E2 or E3 are lvalue bit-fields(until C++11)glvalue bit-fields of the same value category(since C++11) and of types cv1 T and cv2 T, respectively, the operands are considered to be of type cv T for the remaining process, where cv is the union of cv1 and cv2.

If E2 and E3 have different types, and any of the following conditions is satisfied, proceed to stage 3:

At least one of E2 and E3 is a (possibly cv-qualified) class type.
Both of E2 and E3 are lvalues of the same type(until C++11)glvalues of the same value category and the same type(since C++11) except for cv-qualification.

Otherwise, proceed to stage 4.

[edit] Stage 3

Attempts are made to form an implicit conversion sequence^[2] from an operand expression X of type TX to a target type related to the type TY of the operand expression Y as follows:

If Y is an lvalue, the target type is TY&, but an implicit conversion sequence can only be formed if the reference would bind directly to an lvalue(until C++11)a glvalue(since C++11).

If Y is an xvalue, the target type is TY&&, but an implicit conversion sequence can only be formed if the reference would bind directly.

(since C++11)

If Y is an rvalue(until C++11)a prvalue(since C++11) or if none of the conversion sequences above can be formed, and at least one of TX and TY is a (possibly cv-qualified) class type:
- If TX and TY are the same class type (ignoring cv-qualification):
  - If TY is at least as cv-qualified as TX, the target type is TY.
  - Otherwise, no conversion sequence is formed.
- Otherwise, if TY is a base class of TX, the target type is TY with the cv-qualifiers of TX.
- Otherwise, the target type is the type of Z, where Z is the value of Y after applying the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions.
Otherwise, no conversion sequence is formed.

Using this process, it is determined whether an implicit conversion sequence can be formed from E2 to the target type determined for the E3, and vice versa.

If no conversion sequence can be formed, proceed to the next stage.
If exactly one conversion sequence can be formed:
- If the conversion sequence is ambiguous, the program is ill-formed.
- Otherwise, that conversion is applied to the chosen operand and the converted operand is used in place of the original operand for the remaining process, and proceed to the next stage.
If both sequences can be formed, the program is ill-formed.

struct A {};
 
struct B : A {};
 
using T = const B;
 
A a = true ? A() : T(); // Y = A(), TY = A, X = T(), TX = const B, Target = const A

[edit] Stage 4

If E2 and E3 are lvalues of the same type, then the result is an lvalue of that type, and is a bit-field if at least one of E2 and E3 is a bit-field.

(until C++11)

If E2 and E3 are glvalues of the same type and the same value category, then the result has the same type and value category, and is a bit-field if at least one of E2 and E3 is a bit-field.

(since C++11)

Otherwise, the result is an rvalue(until C++11)a prvalue(since C++11).

If E2 and E3 do not have the same type, and either has (possibly cv-qualified) class type, proceed to stage 5.
Otherwise, proceed to stage 6.

[edit] Stage 5

Overload resolution is performed using the built-in candidates to attempt to convert the operands to built-in types:

If the overload resolution fails, the program is ill-formed.
Otherwise, the selected conversions are applied and the converted operands are used in place of the original operands for the remaining process. Proceed to the next stage.

[edit] Stage 6

The array-to-pointer and function-to-pointer conversions are applied to (possibly-converted) E2 and E3. After those conversions, at least one of the following conditions must hold, otherwise the program is ill-formed:

E2 and E3 have the same type. In this case, the result is of that type and the result is copy-initialized using the selected operand.
Both E2 and E3 have arithmetic or enumeration type. In this case, usual arithmetic conversions are applied to bring them to their common type, and the result is of that type.
At least one of E2 and E3 is a pointer. In this case, lvalue-to-rvalue, pointer, function pointer(since C++17) and qualification conversions are applied to bring them to their composite pointer type, and the result is of that type.
At least one of E2 and E3 is a pointer to member. In this case, lvalue-to-rvalue, pointer-to-member, function pointer(since C++17) and qualification conversions are applied to bring them to their composite pointer type, and the result is of that type.

Both E2 and E3 are null pointer constants, and at least one of which is of type std::nullptr_t. In this case, the result is of type std::nullptr_t.

(since C++11)

int* intPtr;
 
using Mixed = decltype(true ? nullptr : intPtr);
 
static_assert(std::is_same_v<Mixed, int*>); // nullptr becoming int*
 
struct A
{
    int* m_ptr;
} a;
 
int* A::* memPtr = &A::m_ptr; // memPtr is a pointer to member m_ptr of A
 
// memPtr makes nullptr as type of pointer to member m_ptr of A
static_assert(std::is_same_v<decltype(false ? memPtr : nullptr), int*A::*>);
 
// a.*memPtr is now just pointer to int and nullptr also becomes pointer to int
static_assert(std::is_same_v<decltype(false ? a.*memPtr : nullptr), int*>);

â Such conditional operator was commonly used in C++11 constexpr programming prior to C++14.
â Member access, whether a conversion function is deleted(since C++11) and whether an operand is a bit-field are ignored.

The result type of a conditional operator is also accessible as the binary type trait std::common_type.

(since C++11) [edit] Overloads

For every pair of promoted arithmetic types L and R and for every type P, where P is a pointer, pointer-to-member, or scoped enumeration type, the following function signatures participate in overload resolution:

LR operator?:(bool, L, R);

P operator?:(bool, P, P);

where LR is the result of usual arithmetic conversions performed on L and R.

The operator â?:â cannot be overloaded, these function signatures only exist for the purpose of overload resolution.

#include <iostream>
#include <string>
 
struct Node
{
    Node* next;
    int data;
 
    // deep-copying copy constructor
    Node(const Node& other)
        : next(other.next ? new Node(*other.next) : NULL)
        , data(other.data)
    {}
 
    Node(int d) : next(NULL), data(d) {}
 
    ~Node() { delete next; }
};
 
int main()
{   
    // simple rvalue example
    int n = 1 > 2 ? 10 : 11;  // 1 > 2 is false, so n = 11
 
    // simple lvalue example
    int m = 10; 
    (n == m ? n : m) = 7; // n == m is false, so m = 7
 
    //output the result
    std::cout << "n = " << n << "\nm = " << m;
}

Output:

[edit] Standard library

Many classes in the standard library overload operator() to be used as function objects.

deletes the object or array
(public member function of std::default_delete<T>) [edit] returns the sum of two arguments
(public member function of std::plus<T>) [edit] returns the difference between two arguments
(public member function of std::minus<T>) [edit] returns the product of two arguments
(public member function of std::multiplies<T>) [edit] returns the result of the division of the first argument by the second argument
(public member function of std::divides<T>) [edit] returns the remainder from the division of the first argument by the second argument
(public member function of std::modulus<T>) [edit] returns the negation of the argument
(public member function of std::negate<T>) [edit] checks if the arguments are equal
(public member function of std::equal_to<T>) [edit] checks if the arguments are not equal
(public member function of std::not_equal_to<T>) [edit] checks if the first argument is greater than the second
(public member function of std::greater<T>) [edit] checks if the first argument is less than the second
(public member function of std::less<T>) [edit] checks if the first argument is greater than or equal to the second
(public member function of std::greater_equal<T>) [edit] checks if the first argument is less than or equal to the second
(public member function of std::less_equal<T>) [edit] returns the logical AND of the two arguments
(public member function of std::logical_and<T>) [edit] returns the logical OR of the two arguments
(public member function of std::logical_or<T>) [edit] returns the logical NOT of the argument
(public member function of std::logical_not<T>) [edit] returns the result of bitwise AND of two arguments
(public member function of std::bit_and<T>) [edit] returns the result of bitwise OR of two arguments
(public member function of std::bit_or<T>) [edit] returns the result of bitwise XOR of two arguments
(public member function of std::bit_xor<T>) [edit] returns the logical complement of the result of a call to the stored predicate
(public member function of std::unary_negate<Predicate>) [edit] returns the logical complement of the result of a call to the stored predicate
(public member function of std::binary_negate<Predicate>) [edit] calls the stored function
(public member function of std::reference_wrapper<T>) [edit] invokes the target
(public member function of std::function<R(Args...)>) [edit] invokes the target
(public member function of std::move_only_function) [edit] invokes the target
(public member function of std::copyable_function) [edit] resumes execution of the coroutine
(public member function of std::coroutine_handle<Promise>) [edit] lexicographically compares two strings using this locale's collate facet
(public member function of std::locale) [edit] compares two values of type value_type
(public member function of std::map<Key,T,Compare,Allocator>::value_compare) [edit] compares two values of type value_type
(public member function of std::multimap<Key,T,Compare,Allocator>::value_compare) [edit] executes the function
(public member function of std::packaged_task<R(Args...)>) [edit] advances the engine's state and returns the generated value
(public member function of std::linear_congruential_engine<UIntType,a,c,m>) [edit] generates the next random number in the distribution
(public member function of std::uniform_int_distribution<IntType>) [edit]

The comma operator is not overloaded by any class in the standard library. The boost library uses operator, in boost.assign, boost.spirit, and other libraries. The database access library SOCI also overloads operator,.

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior CWG 446 C++98 it was unspecified whether a temporary is created for an
lvalue-to-rvalue conversion on the conditional operator always creates a temporary if
the operator returns a class rvalue CWG 462 C++98 if the second operand of a comma operator is a temporary,
it was unspecified whether its lifetime will be extended when
the result of the comma expression is bound to a reference the result of the comma expression
is the temporary in this case
(hence its lifetime is extended) CWG 587 C++98 when the second and third operands of a conditional
operator are lvalues of the same type except for
cv-qualification, the result was an lvalue if these
operands have class types or an rvalue otherwise the result is always
an lvalue in this case CWG 1029 C++98 the type of a destructor call was unspecified specified as void CWG 1550 C++98 parenthesized throw expressions were not allowed in
conditional expressions if other operand is non-void accepted CWG 1560 C++98 void operand of conditional operators caused
gratuitous lvalue-to-rvalue conversion on the
other operand, always resulting in rvalue a conditional expression
with a void can be lvalue CWG 1642 C++98 the expression function in a function call
expression could be a function pointer lvalue not allowed CWG 1805 C++98 when determining the target type for the implicit conversion
sequence, the way to convert Y to Z was unclear made clear CWG 1895 C++98
C++11 unclear if deleted (C++11) or inaccessible (C++98)
conversion function prevents conversion in
conditional expressions, and conversions from base
class to derived class prvalue were not considered handled like
overload resolution CWG 1932 C++98 same-type bit-fields were missing in conditional expressions handled by underlying types CWG 2226 C++11 when determining the target type of the other
operand of a conditional operator, reference could
not bind to an xvalue if that operand is an lvalue allowed CWG 2283 C++17 the type completeness requirement for function call
operator was accidently removed by P0135R1 restored the requirement CWG 2321 C++98 when determining the target type of the other operand
of a conditional operator, a derived class type could
not be converted to a less cv-qualified base class type allowed to convert to the base
class type with the cv-qualification
from the derived class operand CWG 2715 C++98 the initialization and destruction of each
parameter would occur within the context of
the calling function, which might not exist^[1] occurs within the context of
the enclosing full-expression CWG 2850 C++98 the destruction order of parameters was unclear made clear CWG 2865 C++98 if TX and TY are the same class type and TX is
more cv-qualified than TY, an implicit conversion
sequence could still be formed from a prvalue Y no conversion sequence
will be formed in this case CWG 2906 C++98 lvalue-to-rvalue conversions were unconditionally applied
in the rvalue result case for the conditional operator only applied in some cases

â For example, functions can be called in the initializer of a namespace-scope variable, there is no âcalling functionâ in this context.

[edit] See also

Operator precedence
Operator overloading

Common operators assignment increment
decrement arithmetic logical comparison member
access other

a = b a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b

++a --a a++ a--

+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b

!a a && b a || b

a == b a != b a < b a > b a <= b a >= b a <=> b

a[...] *a &a a->b a.b a->*b a.*b

function call

a(...)

comma

a, b

conditional

a ? b : c

Special operators

static_cast converts one type to another related type
dynamic_cast converts within inheritance hierarchies
const_cast adds or removes cv-qualifiers
reinterpret_cast converts type to unrelated type
C-style cast converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast
new creates objects with dynamic storage duration
delete destructs objects previously created by the new expression and releases obtained memory area
sizeof queries the size of a type
sizeof... queries the size of a pack (since C++11)
typeid queries the type information of a type
noexcept checks if an expression can throw an exception (since C++11)
alignof queries alignment requirements of a type (since C++11)

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