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

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

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

std::ref is a 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

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).

Another place where you may need std::ref is when passing objects to threads where you want each thread to operate on the single object and not a copy of the object.

int main(){
BoundedBuffer buffer(200);

std::thread c1(consumer, 0, std::ref(buffer));
std::thread c2(consumer, 1, std::ref(buffer));
std::thread c3(consumer, 2, std::ref(buffer));
std::thread p1(producer, 0, std::ref(buffer));
std::thread p2(producer, 1, std::ref(buffer));

c1.join();
c2.join();
c3.join();
p1.join();
p2.join();

return 0; }

where you wish various functions running in various threads to share a single buffer object. This example was stolen from this excellent tutorial ( C++11 Concurrency Tutorial - Part 3: Advanced locking and condition variables (Baptiste Wicht) ) (hope I did the attribution correctly)