SetClassLongPtrA function (winuser.h)

Replaces the specified value at the specified offset in the extra class memory or the WNDCLASSEX structure for the class to which the specified window belongs.

Note  To write code that is compatible with both 32-bit and 64-bit Windows, use SetClassLongPtr. When compiling for 32-bit Windows, SetClassLongPtr is defined as a call to the SetClassLong function



ULONG_PTR SetClassLongPtrA(
  [in] HWND     hWnd,
  [in] int      nIndex,
  [in] LONG_PTR dwNewLong


[in] hWnd

Type: HWND

A handle to the window and, indirectly, the class to which the window belongs.

[in] nIndex

Type: int

The value to be replaced. To set a value in the extra class memory, specify the positive, zero-based byte offset of the value to be set. Valid values are in the range zero through the number of bytes of extra class memory, minus eight; for example, if you specified 24 or more bytes of extra class memory, a value of 16 would be an index to the third integer. To set a value other than the WNDCLASSEX structure, specify one of the following values.

Value Meaning
Sets the size, in bytes, of the extra memory associated with the class. Setting this value does not change the number of extra bytes already allocated.
Sets the size, in bytes, of the extra window memory associated with each window in the class. Setting this value does not change the number of extra bytes already allocated. For information on how to access this memory, see SetWindowLongPtr.
Replaces a handle to the background brush associated with the class.
Replaces a handle to the cursor associated with the class.
Replaces a handle to the icon associated with the class.
Retrieves a handle to the small icon associated with the class.
Replaces a handle to the module that registered the class.
Replaces the pointer to the menu name string. The string identifies the menu resource associated with the class.
Replaces the window-class style bits.
Replaces the pointer to the window procedure associated with the class.

[in] dwNewLong


The replacement value.

Return value


If the function succeeds, the return value is the previous value of the specified offset. If this was not previously set, the return value is zero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.


If you use the SetClassLongPtr function and the GCLP_WNDPROC index to replace the window procedure, the window procedure must conform to the guidelines specified in the description of the WindowProc callback function.

Calling SetClassLongPtr with the GCLP_WNDPROC index creates a subclass of the window class that affects all windows subsequently created with the class. An application can subclass a system class, but should not subclass a window class created by another process.

Reserve extra class memory by specifying a nonzero value in the cbClsExtra member of the WNDCLASSEX structure used with the RegisterClassEx function.

Use the SetClassLongPtr function with care. For example, it is possible to change the background color for a class by using SetClassLongPtr, but this change does not immediately repaint all windows belonging to the class.


The winuser.h header defines SetClassLongPtr as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header winuser.h (include Windows.h)
Library User32.lib
DLL User32.dll
API set ext-ms-win-ntuser-windowclass-l1-1-2 (introduced in Windows 10, version 10.0.10240)

See also







Window Classes