How to handle lifecycle of dynamically allocated data in Windows messages?

Question

Simple task: Send a windows message with dynamically allocated data, e.g. an arbitrary length string. How would you manage the responsibility to free this data?

The receiver(s) of the windows message could be responsible to free this data. But: How can you guarantee that all messages will actually be received and thus the linked data will be freed? Imagine the situation that the receiver is shutting down, so it won't process it's message queue any more. However, the message queue still exists (for some time) and can still accept messages, which won't be processed any more.

Thanks!

Answer 1

PostMessage returns a BOOL that tells you whether the message was posted or not. This is usually good enough, because your window should be valid until it receives the WM_DESTROY and the WM_NCDESTROY messages. After a call to DestroyWindow (which sends these messages) you should not be able to successfully call PostMessage again.

Now, if your PostMessage returns FALSE you have to clean up. If it doesn't, the window procedure has to clean up. Don't send messages that have to be cleaned up to random windows that might not handle them. Actually, don't send any WM_USER + x messages to any windows you don't handle.