GWLPWNDPROC

GWLP_WNDPROC is a constant used with the Windows API to access or replace the window procedure associated with a specific window. It identifies the slot in the per-window user data where a pointer to the window procedure (WNDPROC) is stored. By using this index with GetWindowLongPtr or SetWindowLongPtr, applications can retrieve the current window procedure or install a new one, enabling window procedure subclassing or custom message handling.

In practice, a programmer commonly subclasses a window by setting a new window procedure and preserving the old one for calls to the original procedure. Typical usage is to call SetWindowLongPtr with GWLP_WNDPROC to install a custom procedure, while saving the previous WNDPROC value. The new procedure should follow the standard WNDPROC signature and may call CallWindowProc to forward messages to the original procedure when appropriate. For example: SetWindowLongPtr(hWnd, GWLP_WNDPROC, (LONG_PTR)MyWndProc); and then in MyWndProc use CallWindowProc to pass unhandled messages to the old procedure.

The value of GWLP_WNDPROC is -4, and it is used with GetWindowLongPtr/SetWindowLongPtr to access the window procedure pointer in both 32-bit and 64-bit environments. On 64-bit systems, SetWindowLongPtr/GetWindowLongPtr are the correct APIs to avoid truncation issues that can occur with the older GetWindowLong/SetWindowLong. Proper handling requires storing the previous procedure and restoring it when subclassing ends, and ensuring the subclass does not interfere with system messages or window destruction.

As an alternative to manual subclassing, modern code can use SetWindowSubclass, which provides safer, more managed subclassing with automatic cleanup.