Explanation of std::function

Question

What is the purpose of std::function? As far as I understand, std::function turns a function, functor, or lambda into a function object.

I

Answer 1

In most uses that I've seen, std::function was overkill. But it serves two purposes.

First, it gives you a uniform syntax for calling function objects. For example, you can use an std::function instantiation to wrap an ordinary function that takes a single argument of a class type or a member function and the class object that it should be applied to without worrying about the different calling syntax.

struct S {
    void f();
};

void g(const S&);

S obj;

typedef std::function functor1(&S::f, obj);
typedef std::function functor2(&g, obj);

functor1(); // calls obj.f()
functor2(); // calls g(obj);

Note that both functors here are called with the same syntax. That's a big benefit when you're writing generic code. The decision of how to call the underlying function is made within the std::function template, and you don't have to figure it out in your code.

The other big benefit is that you can reassign the function object that a std::function object holds:

functor1 = std::function()>(&g, obj);

This changes the behavior of functor1:

functor1() // calls g(obj)

Sometimes that matters.