It's magic! std::bind

'std::bind' is a powerful helper for working around 'std::function'.

'std::bind' makes a instance of 'std::function' within user-defined parameter.

You don't need to consider how does 'std::bind' work, just learn how to use it.

Anyway, let's have a look at examples.

#include <iostream>
#include <functional>

std::function<void()> func;

void no_arg()
{
  std::cout << "no_arg\n";
}

void int_arg(int i)
{
  std::cout << "int_arg " << i << std::endl;
}

class Foo
{
  public:
  void no_arg_method() {
    std::cout << "no_arg_method\n";
  }
};

int main()
{
  // func and no_arg are both take no arguments, so we can assign no_arg to func directry
  func = &no_arg;
  func();
  
  // int_arg takes one int argument, so we need to use std::bind to make new function object
  // with int_arg taking one argument of 10.
  func = std::bind(&int_arg, 10);
  func(); // no arguments provided here.
  
  // Foo::no__arg_method takes no arguments, but it needs implicit argument of 'this',
  // std::bind can bind instance address like this.
  Foo foo;
  func = std::bind(&Foo::no_arg_method, &foo);
  func(); // calls foo's no_arg_method

}

repl.it

And here is one more example, with function object taking arguments.

#include <iostream>
#include <functional>

std::function<void(int i)> func;

void int_arg(int i)
{
  std::cout << "int_arg " << i << std::endl;
}

void int_arg2(int i, int j)
{
  std::cout << "int_arg2 " << i << "/" << j << std::endl;
}

class Foo
{
  public:
  void int_arg_method(int i) {
    std::cout << "int_arg_method " << i << std::endl;
  }
};

int main()
{
  // func and no_arg are both take one int argument, so we can assign int_arg to func directry
  func = &int_arg;
  func(20); // call with argument;
  
  // int_arg2 takes two int argument, so we need to use std::bind to make new function object
  // with int_arg2 taking one argument(10 for instance).
  // std::placeholders::_1 means that 'will be replaced by first argument by caller' 
  func = std::bind(&int_arg2, 10, std::placeholders::_1);
  func(20); // std::placeholders::_1 is replaced to 20
  
  // also we can use placeholders with class method.
  Foo foo;
  func = std::bind(&Foo::int_arg_method, &foo, std::placeholders::_1);
  func(20);

}

repl.it