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Showing content from https://cplusplus.com/reference/functional/ref/ below:

function template

<functional>

std::ref reference (1) 
template <class T> reference_wrapper<T> ref (T& elem) noexcept;
copy (2) 
template <class T> reference_wrapper<T> ref (reference_wrapper<T>& x) noexcept;
move (3) 
 template <class T> void ref (const T&&) = delete;

Construct reference_wrapper

Constructs an object of the appropriate reference_wrapper type to hold a reference to elem.

If the argument is itself a reference_wrapper (2), it creates a copy of x instead.

The function calls the proper reference_wrapper constructor.



Parameters
elem
An lvalue reference, whose reference is stored in the object.
x
A reference_wrapper object, which is copied.

Return Value A reference_wrapper object of the appropriate type to hold an element of type T.

Example 
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// ref example
#include <iostream>     // std::cout
#include <functional>   // std::ref

int main () {
  int foo (10);

  auto bar = std::ref(foo);

  ++bar;

  std::cout << foo << '\n';

  return 0;
}

Output:


Data races The initialization version (1) does not access elem, but the returned object can be used to access or modify it.
The copying version (2) accesses its argument (x), returning an object that which can be used to access or modify its referred element.

Exception safetyNo-throw guarantee: never throws exceptions.

See also
reference_wrapper::reference_wrapper
Construct reference wrapper (public member function)
reference_wrapper::operator()
Access element (functional form) (public member function)
cref
Construct reference_wrapper to const (function template)

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