Reference Wrapper in C++

std::reference_wrapper is a class template in C++ that allows you to store references to objects or functions in a way that makes them copyable and assignable. Normally, C++ references can't be stored in standard containers like std::vector or std::list, because references are not copyable. std::reference_wrapper<T> solves this problem by internally storing a pointer to the referenced object.

It acts like a wrapper around a reference and behaves almost like the original object. It can be passed to functions that take T& (a reference to T), because std::reference_wrapper<T> is implicitly convertible to T&.

This is especially useful when you want to store and work with references in STL containers or when passing around callable objects like functions or lambdas.

Example of reference_wrapper Class

In this C++ example, we demonstrate how std::reference_wrapper<int> enables storing references to variables (a, b, and c) in a std::vector. By using reference_wrapper:

#include <iostream>
#include <vector>
#include <functional>

int main() {
   int a = 10, b = 20, c = 30;
   std::vector < std::reference_wrapper < int >> refVec = {a, b, c};

   for (auto & ref: refVec) {
      // Modify original values
      ref.get() += 1;
   }

   std::cout << a << " " << b << " " << c << std::endl;
}

Following is the output of the above code ?

11 21 31

Creating Character Array Using reference_wrapper Class

Following is another C++ example, to demonstrate the use of std::reference_wrapper by creating an array of character "references" ?

#include <iostream>
#include <functional>
using namespace std;

int main() {
   char a = 't', b = 'u', c = 't', d = 'o', e = 'r', f = 'i', g = 'a', h = 'l', i = 's';

   reference_wrapper < char > ref[] = {
      a,
      b,
      c,
      d,
      e,
      f,
      g,
      h,
      i
   };

   for (char & str: ref)
      cout << str;
   return 0;
}

Following is the output ?

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