Chapter 42. Boost.Ref

The library Boost.Ref provides two functions, boost::ref() and boost::cref(), in the header file boost/ref.hpp. They are useful if you use, for example, std::bind() for a function which expects parameters by reference. Because std::bind() takes parameters by value, you have to deal with references explicitly.

Boost.Ref was added to the standard library in C++11, where you will find the functions std::ref() and std::cref() in the header file functional.

#include <boost/ref.hpp>
#include <vector>
#include <algorithm>
#include <functional>
#include <iostream>

void print(std::ostream &os, int i)
{
  os << i << std::endl;
}

int main()
{
  std::vector<int> v{1, 3, 2};
  std::for_each(v.begin(), v.end(),
    std::bind(print, boost::ref(std::cout), std::placeholders::_1));
}

In Example 42.1, the function print() is passed to std::for_each() to write the numbers in v to an output stream. Because print() expects two parameters – an output stream and the number to be written – std::bind() is used. The first parameter passed to print() through std::bind() is std::cout. However, print() expects a reference to an output stream, while std::bind() passes parameters by value. Therefore, boost::ref() is used to wrap std::cout. boost::ref() returns a proxy object that contains a reference to the object passed to it. This makes it possible to pass a reference to std::cout even though std::bind() takes all parameters by value.

The function template boost::cref() lets you pass a const reference.