Functor reference through a std::function

Question

Basically, I would like to have the following semantic :

#include <functional> #include <iostream>  class test {   public:     void add(std::function<void()> f)     {       f();     }      void operator()()     {       ++x;     }      int x = 33; };  int main() {   test t;   t.add(t);   // wanted: x == 34 instead: x == 33 since add(t) copies it } 

I understand std::function wraps a copy of the callable object but is there any way to get a reference to a callable object using std::function?

Answer 1

You want to use the std::ref template function to create a reference wrapper for your instance:

std::reference_wrapper is a class template that wraps a reference in a copyable, assignable object.

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.

You would use it like this:

t.add(std::ref(t)); 

Answer 2

Another approach would be to pass in a lambda - binding t as a reference:

t.add( [&t]{ t(); } ); 

This seems more readable to me (but only because I'm familiar with lambdas and not std::bind). The distinction is perhaps reminiscent of an item in Scott Meyers' Effective Modern C++, #34: Prefer Lambdas to std::bind.