std::ref,std::cref (3) Linux Manual Page

std::ref,std::cref – std::ref,std::cref

Synopsis

Defined in header<functional>

    template <class T>
    (1)(since C++ 11)

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

template <class T>
(2)(since C++ 11)

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

template <class T>
(3)(since C++ 11)

    void ref(const T &&) = delete;

template <class T>
(4)(since C++ 11)

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

template <class T>
(5)(since C++ 11)

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

template <class T>
(6)(since C++ 11)

    void cref(const T &&) = delete;

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