Home Original page

std::ref, std::cref - cppreference.com

From cppreference.com

Defined in header <functional>

template< class T > std::reference_wrapper<T> ref( T& t ) noexcept;

 (1) (since C++11)
(constexpr since C++20)

template< class T > std::reference_wrapper<T> ref( std::reference_wrapper<T> t ) noexcept;

 (2) (since C++11)
(constexpr since C++20)

template< class T > void ref( const T&& ) = delete;

 (3) (since C++11)

template< class T > std::reference_wrapper<const T> cref( const T& t ) noexcept;

 (4) (since C++11)
(constexpr since C++20)

template< class T > std::reference_wrapper<const T> cref( std::reference_wrapper<T> t ) noexcept;

 (5) (since C++11)
(constexpr since C++20)

template< class T > void cref( const T&& ) = delete;

 (6) (since 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

#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