std::ref, std::cref

 

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unary_function (until C++17) binary_function (until C++17) ptr_fun (until C++17) pointer_to_unary_function (until C++17) pointer_to_binary_function (until C++17) mem_fun (until C++17) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17)(until C++17)(until C++17)(until C++17) not1 (until C++20) not2 (until C++20)	binder1stbinder2nd (until C++17)(until C++17) bind1stbind2nd (until C++17)(until C++17) mem_fun_ref (until C++17) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17)(until C++17)(until C++17)(until C++17) unary_negate (until C++20) binary_negate (until C++20)

 

Defined in header <functional>
template< class T > std::reference_wrapper<T> ref(T& t) noexcept;	(1)	(since C++11)
template< class T > std::reference_wrapper<T> ref( std::reference_wrapper<T> t ) noexcept;	(2)	(since C++11)
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)
template< class T > std::reference_wrapper<const T> cref( std::reference_wrapper<T> t ) noexcept;	(5)	(since C++11)
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) ref(t.get())

4) std::reference_wrapper<const T>(t)

5) cref(t.get())

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

See also

reference_wrapper (C++11)

CopyConstructible and CopyAssignable reference wrapper (class template)