Boost.Bind | C++

Introduction

boost::bind is a generalization of the standard functions std::bind1st and std::bind2nd. It supports arbitrary function objects, functions, function pointers, and member function pointers.

Syntax

Below syntax bind function with parameters.

boost::bind(namespace::functionName, _1, _N);

To bind a member function, an instance of an object on which the member function can be called must be provided to bind. This is because a member function has access to class data and if not called from an instance any access to class data would result in undefined behaviour.

boost::bind(&Class::functionName, objPtr); // no parameters 
boost::bind(&Class::functionName, objPtr, _1, _N); // with parameters

Bind with functions

In the below example, bind(sum, 1, 2) will produce a function object that takes no arguments and returns sum(1, 2).

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <iostream>


int sum(int a, int b)
{
  return a + b;
}

int main()
{
  boost::function<int(int x, int y)> callback;
  callback = boost::bind(sum, 1, 2);

  std::cout << callback(10,2) << std::endl; // 3
  std::cout << callback(1, 2) << std::endl; // 3
}

To bind arguments, use placeholder. Placeholders denote the arguments that are to be supplied to the resulting function object, and Boost.Bind supports up to nine such arguments. The placeholders are called _1, _2, _3, _4, and so on up to _9.

In the below example,  bind(sum, _1, 2)(x) is equivalent to sum(x, 5); where _1 is a placeholder argument.

int sum(int a, int b)
{
  return a + b;
}

int main()
{
  boost::function<int(int x, int y)> callback;
  callback = boost::bind(sum, 1, 2);

  std::cout << callback(10,2) << std::endl; // 3
  std::cout << callback(1, 2) << std::endl; // 3
}

Bind with function objects

bind can also be used function objects. Return type of the generated function object’s operator() has to be specified explicitly.

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <iostream>

int main()
{
  struct mathOps
  {
      int operator()(int a, int b) { 
            return a - b; }

      bool operator()(long a, long b) { 
          return a == b; 
      }
  };

  mathOps f;
  int x = 104;
  int y = 100;

  std::cout << boost::bind<int>(f, _1, _2)(x, y) << std::endl;  // 4
  std::cout << boost::bind<bool>(f, _1, _2)(x, y) << std::endl; // 1
}

// Below example shows argument substitution mechanism.
bind(sum, _2, _1)(x, y);      // sum(y, x)
bind(sum, _1, _1)(x, y);      // sum(x, x)

Bind with pointers to members

bind can also be used with pointer to member function as shown in below example

int main()
{
  struct mathOps
  {
    int sum(int a, int b) { 
      return a - b; }
  };

  mathOps x;
  int i = 5;
  int j = 15;

  std::cout << boost::bind(&mathOps::sum, &x, _1, _2)(i, j);  // (&x)->f(i)
  std::cout << boost::bind(&mathOps::sum, x, _2, _2)(i, j);   // (internal copy of x).f(i)
}

Reference

How To Use Boost.Bind