What would be a "Hello, World!" example for "std::ref"?

Question

Can somebody give a simple example which demonstrates the functionality of std::ref? I mean an example in which some other constructs (like tuples, or data type templates) are used only if it is impossible to explain std::ref without them.

I found two questions about std::ref here and here. But in the first one it goes about a bug in a compiler and in the second one, examples of use of std::ref do not contain std::ref and they involve tuples and data type templates which make understanding of these examples complex.

Answer 1

You should think of using std::ref when a function:

• takes a template parameter by value
• copies/moves a template parameter, such as std::bind or the constructor for std::thread.

std::ref creates a copyable value type that behaves like a reference.

This example makes demonstrable use of std::ref.

#include <iostream> #include <functional> #include <thread>  void increment( int &x ) {   ++x; }  int main() {   int i = 0;    // Here, we bind increment to a COPY of i...   std::bind( increment, i ) ();   //                        ^^ (...and invoke the resulting function object)    // i is still 0, because the copy was incremented.   std::cout << i << std::endl;    // Now, we bind increment to std::ref(i)   std::bind( increment, std::ref(i) ) ();   // i has now been incremented.   std::cout << i << std::endl;    // The same applies for std::thread   std::thread( increment, std::ref(i) ).join();   std::cout << i << std::endl; } 

Output:

0 1 2 

Answer 2

void PrintNumber(int i) {...}  int n = 4; std::function<void()> print1 = std::bind(&PrintNumber, n); std::function<void()> print2 = std::bind(&PrintNumber, std::ref(n));  n = 5;  print1(); //prints 4 print2(); //prints 5 

std::ref is mainly used to encapsulate references when using std::bind (but other uses are possible of course).