SetWindowLongPtr

SetWindowLongPtr is a Windows API function that stores or retrieves a per-window value in a manner that is safe for 64-bit applications. It is commonly used to subclass a window by replacing its window procedure or to associate application data with a window. On 64-bit Windows, SetWindowLongPtr provides a pointer-sized alternative to the older SetWindowLong function, which helps maintain compatibility and correctness across architectures.

The function takes three parameters: a handle to the window (HWND hWnd), an index specifying which value to set (int nIndex), and the new value to store (LONG_PTR dwNewLong). The nIndex parameter is typically one of the GWLP_ constants for long-pointer values, with GWLP_WNDPROC used to replace the window procedure and GWLP_USERDATA used to store a user-defined pointer or data. Other GWLP_* values can be used to store additional per-window information as appropriate. In contrast, some older uses relied on GWL_ constants; SetWindowLongPtr and GetWindowLongPtr adapt these values for 64-bit safety.

The return value is the previous value stored at the specified slot. If the function fails, the return value is zero, and extended error information can be obtained by calling GetLastError. When subclassing a window, the previous window procedure is returned; you should typically forward messages to the original procedure via CallWindowProc.

Compatibility notes: SetWindowLongPtr replaces SetWindowLong on 64-bit systems. For 32-bit code, SetWindowLongPtr is usually defined to map to SetWindowLong. Use the GWLP_* indices to avoid truncation and to ensure correct behavior in 64-bit environments.

See also: GetWindowLongPtr, GetWindowLong, SetWindowLong, GWLP_WNDPROC, GWLP_USERDATA, CallWindowProc.