GWLPHINSTANCE

GWLP_HINSTANCE, often written as GWLPHINSTANCE, is a Windows API constant used with the GetWindowLongPtr and SetWindowLongPtr functions to access the HINSTANCE value associated with a window. It is part of the GWLP (GetWindowLongPtr) family of indices that refer to the extra window memory allocated for each window via the window class or window creation. The HINSTANCE represents the module instance that owns the window and is typically used to identify resources or perform module-specific initialization for that window.

Usage and behavior: When calling GetWindowLongPtr with hWnd and GWLP_HINSTANCE, the function returns the HINSTANCE for the window, allowing a program to retrieve the module handle that created or manages the window. Conversely, SetWindowLongPtr can store an HINSTANCE or related value at that index, though changing the HINSTANCE is uncommon and may have implications for resource loading and message handling. The index is designed to be portable between 32-bit and 64-bit applications, using LONG_PTR to accommodate pointer-sized data.

Relation to other APIs: GWLP_HINSTANCE is the 64-bit/32-bit compatible counterpart to the older GWL_HINSTANCE used with GetWindowLong. Modern code typically uses GetWindowLongPtr and SetWindowLongPtr with GWLP indices to access per-window data, including the HINSTANCE, to ensure correct behavior on 64-bit platforms.

Notes: The value is part of the Windows API defined in WinUser.h. Proper usage requires matching the data type to LONG_PTR and awareness of window lifecycle, since the HINSTANCE may be used for resource loading and may be tied to the window’s creation and class registration.