Win32 window Owner vs window Parent?
WinapiWindowParentOwnerWinapi Problem Overview
In Win32 programming, what is the difference between a window's parent and a window's owner? I thought I had it figured out, then I came across this code:
SetWindowLong(handle, GWL_HWNDPARENT, foo);
This actually sets the window's owner, not the parent - despite the GWL_HWNDPARENT being used. Are the terms parent/owner interchangeable, or is there actually a difference?
Winapi Solutions
Solution 1 - Winapi
Ownership is a relationship between two top level windows while Parent is a relationship between a top level and a WS_CHILD, or a WS_CHILD and another WS_CHILD.
The parent of a button is the form it is on, while a message box is owned by the form that showed it.
Read this article from Microsoft Win32 Window Hierarchy and Styles to get a much clearer understanding of Ownership, Parenting, ZOrder, SetWindowLong, GetWindow and all the other nasty bits of the Win32 api for creating window relationships.
EDIT: Looks like Microsoft took down that content, here is another reasonable summary of Ownership / Parenting.
Solution 2 - Winapi
Owner is the Window* responsible for a control or dialog (for example, responsible for creating/destroying the window).
Parent is the next-senior window* to a control or dialog in the window chain, but isn't actually responsible for it (doesn't necessarily care about its lifecycle, etc). A window's parent can also be its owner.
*Window vs window: Window is an actual window displayed on the screen; window is any object with a HWND (includes buttons, panels, etc).
Solution 3 - Winapi
Chen's blog post is the one to read. The key point for me is that the WS_CHILD style must be used on the child window. You can attempt to create a child window and pass the parent handle in to CreateWindow( ), but if you don't have the WS_CHILD style set the two windows will have the owner relationship, not the parent/child relationship.
Solution 4 - Winapi
It's super easy: the code is wrong. End of story right here.
Yes, some windows may happen to react favorably to such a call - someone not knowing any better may have implemented support for it. Quoth documentation (and it's old documentation) - You must not call SetWindowLong with the GWL_HWNDPARENT index to change the parent of a child window. Instead, use the SetParent function.
So, all there's to it: you came upon buggy code, change it to SetParent or refactor to do something else, and keep going?