What is LRESULT? How do I use it?

I am using a message map with a custom ON_MESSAGE handler. The callback function called by the message handler requres the return type to be LRESULT, but I am not clear on understanding what I should return in the LRSULT, and how the return value is used.

my callback function called by the ON_MESSAGE call in the message map calls a function like:
LRESULT ClassName::CallbackFunction(WPARAM wParam, LPARAM lParam);
and I have found many examples online that return "LRESULT(true)" from this function, but little explanation of why return true. Could I/Should I return other values ever? What does this return value get used for?

MSDN's explanation that LRESULT is the "Signed result of message processing." and is typdef'd from LONG_PTR doesn't give me the insight I was hoping for. Thanks for your help!

It depends on what you are trying to do, but if you are using it from ON_MESSAGE the returned values usually doesn't matter. return 0L; should do fine.

If you are using a custom message, then Quell's advice is OK, but if you are handing a message normally handled by the default windows proc, then you should return TRUE to indicate that you handled the message and the default proc need not bother; otherwise, the default windows proc will still process it and could override anything you already did.

You should read what Windows expects for each message you want to handle. For some of them it expects 0, for others - something else.