Call a non-static method from no object using std::function - c++

Question

While doing some random-ish, weird-ish things to understand a little bit more C++ (just to say I'm still a newbie), I came across something I can't properly understand : I'm currently using SFML which provides a few methods to create and manage a window, namely this method to close it: void sf::RenderWindow::close(). It doesn't take any argument, and it can't be called directly but through an instantiated object.
sf::RenderWindow::close(); // error
My problem is the following code :
sf::RenderWindow window(sf::VideoMode(800, 600), "test"); // added after edit
const std::function <void(sf::RenderWindow &)> callback(sf::RenderWindow::close);
// callback();
callback(window); // closes the window
I'm not sure to understand what's happening here as I'm not using any object... even anonymous at first sight... I guess...
If someone can enlighten me, I'd really appreciate it.
Thanks you.
Edit:
My mistake, yes there's an object.
#include <iostream>
#include <functional>
#include <SFML/Graphics.hpp>
int main()
{
sf::RenderWindow window(sf::VideoMode(800, 600), "test");
const std::function <void(sf::RenderWindow &)> call(sf::RenderWindow::close);
call(window);
return 0;
}
Actually, I understand this as if it were sf::RenderWindow::close(window), similarly to metatables in lua I suppose.

Answer 1

It's linked to the concept of Callable:
Since the stored target of the std::function (i.e. sf::RenderWindow::close) is a pointer to member function and the first argument (i.e. window) is a (reference to) an object of type RenderWindow, then the invocation of the function object is equivalent to window.close().
You could also have written something along those lines:
std::function<void(sf::RenderWindow*)> call = &sf::RenderWindow::close;
call(&window);
Below a code sample that doesn't involve SFML:
#include <iostream>
#include <functional>
class Window {
public:
void close() {std::cout << "close" << std::endl; }
};
int main(int argc, const char * argv[]) {
Window w;
std::function<void(Window&)> f = &Window::close;
f(w);
std::function<void(Window*)> g = &Window::close;
g(&w);
return 0;
}