NAME¶

std::ref,std::cref - std::ref,std::cref

Synopsis¶

Defined in header <functional>
template< class T > (since
std::reference_wrapper<T> ref( T& t ) C++11)
noexcept; (until
C++20)
template< class T > (since
constexpr std::reference_wrapper<T> ref( T& C++20)
t ) noexcept;
template< class T >
(since
std::reference_wrapper<T> C++11)
(until
ref( std::reference_wrapper<T> t ) C++20)
noexcept;
template< class T >

constexpr std::reference_wrapper<T> (since
C++20)
ref( std::reference_wrapper<T> t )
noexcept;
template< class T > (3) (since
void ref( const T&& ) = delete; (1) C++11)
template< class T > (since
std::reference_wrapper<const T> cref( const C++11)
T& t ) noexcept; (until
(2) C++20)
template< class T > (since
constexpr std::reference_wrapper<const T> C++20)
cref( const T& t ) noexcept;
template< class T >
(since
std::reference_wrapper<const T> C++11)
(4) (until
cref( std::reference_wrapper<T> t ) C++20)
noexcept;
template< class T > (5)

constexpr std::reference_wrapper<const T> (since
C++20)
cref( std::reference_wrapper<T> t )
noexcept;
template< class T > (6) (since
void cref( const T&& ) = delete; C++11)

Function templates ref and cref are helper functions that generate an object of type
std::reference_wrapper, using template argument deduction to determine the template
argument of the result.

T may be an incomplete type. (since C++20)

Parameters¶

t - lvalue reference to object that needs to be wrapped or an instance of
std::reference_wrapper

Return value¶

1) std::reference_wrapper<T>(t)
2) t
4) std::reference_wrapper<const T>(t)
5) t
3,6) rvalue reference wrapper is deleted.

Example¶

// Run this code

#include <functional>
#include <iostream>

void f(int& n1, int& n2, const int& n3)
{
std::cout << "In function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
++n1; // increments the copy of n1 stored in the function object
++n2; // increments the main()'s n2
// ++n3; // compile error
}

int main()
{
int n1 = 1, n2 = 2, n3 = 3;
std::function<void()> bound_f = std::bind(f, n1, std::ref(n2), std::cref(n3));
n1 = 10;
n2 = 11;
n3 = 12;
std::cout << "Before function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
bound_f();
std::cout << "After function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
}

Output:¶

Before function: 10 11 12
In function: 1 11 12
After function: 10 12 12

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 3146 C++11 unwrapping overloads sometimes led to error made always valid

See also¶

reference_wrapper CopyConstructible and CopyAssignable reference wrapper
(C++11) (class template)