Question

I’ve a function callback to handle the window’s events like WM_SIZE. Now, how to pass the function callback to the open window? (It’s for a command-line application).

I’ve seen RegisterClassEx but I think that it’s used at creating a new window.

Answer 1

If you want to handle windows messages for an existing window you have a couple options:

Subclassing. Each window has a “window proc” callback function (that you are referring to), and you can replace it using SetWindowLongPtr with GWLP_WNDPROC. Then in your own window proc, you need to forward messages to the original one that you replaced. There are some limitations though:

1. You cannot safely un-subclass a window.
2. You can only do this within your own process.

Windows hooks, using SetWindowsHookEx. Use this if you want to peek into the messages of other windows – you can see all messages for windows in a given thread, or system-wide. There are different types of hooks that allow you to catch different types of messages. This is a pretty heavy approach; a last resort so you don’t bog down the system unnecessarily.