How to declare static C++ function as friend on OSX

Question

I've built an application twice: once in Visual Studio and another time in XCode. One of the libraries I used, GLFW, allows you to use the glfwSetWindowSizeCallback function to detect resizing of the window.

My window class, Window , has two private members, width and height. And upon the calling of my callback, window_size_callback , I wanted the values of width and height to be updated. However, I wanted to do this without the use of setters.

So, I made window_size_callback a static friend. This solution worked just fine in the Visual Studio compiler; however, XCode returned an error: 'static' is invalid in friend declarations.

window_size_callback :

void window_size_callback(GLFWwindow* window, int width, int height) {
    Window* win = (Window*)glfwGetWindowUserPointer(window);
    win->width = width;
    win->height = height;
}

glfwGetWindowUserPointer is used to get the current window instance from outside of the class.

header file:

#include <GLFW/glfw3.h>

class Window {
private:
    int m_width;
    int m_height;
private:
    friend static void window_size_callback(GLFWwindow* window, int width, int height);
}

Without the friend keyword, window_size_callback is unable to access these members.

Why is VS fine with this and XCode not?

And, how can I get around this without using setters?

Answer 1

Just remove the static . It makes no sense as I explained in the comments. Here's a snippet that should clear things:

class Window {
private:
    int m_width;
    int m_height;
private:
    friend void window_size_callback(GLFWwindow*, int, int);
};

// as you can see 'window_size_callback' is implemented as a free function
// not as a member function which is what 'static' implies
void window_size_callback(GLFWwindow* window, int width, int height) {
    Window* win = (Window*)glfwGetWindowUserPointer(window);
    win->width = width;
    win->height = height;
}

A friend function cannot be a static member of a class. I'm guessing VS allows the syntax as an extension. Don't count on it.