C/C++ - Windows - how to keep track of HWND context data

Question

In Windows, suppose you have multiple windows (HWNDs) of the same window class open. How do you keep track of the context data in the window procedure so that, for example, window 1 does not get modified when the user tried to type in window 2?

CreateWindow() does not return until after the WndProc() has been called several times, so you can't simply set the resulting HWND to the context data and do a lookup in the WndProc(); you do need to set it in the WndProc().

WndProc() doesn't directly have the context information passed to it except on window creation messages, but unfortunately window creation messages aren't exactly the first messages to be passed to WndProc(). Nay, I find things such as WM_SIZE, WM_NCSIZE, and even some others are passed before I ever see WM_CREATE.

Storing the HWND in a linked list type of storage mechanism would be inefficient with large amounts of windows: each control in a window is simply another type of window and therefore another HWND of which you need to keep track; after a few hundred controls, searching the linked list for the HWND will be a major bottleneck in the program after a few dozen messages are passed to the program in a short amount of time!

From what I hear, some people use SetWindowLong() - but I also hear that some libraries like to use that too to store their own context information separate from the program and that window data collisions can sometimes occur. How can that be avoided?

I'm not following this.. do you want to differentiate between your windows? Why would window1 be modified when typing into window2? — default
SetWindowLongPtr(GWL_USERDATA) is the way to go. Libraries should NOT use that unless they're creating the window. The window's user data belongs to whoever created the window -- if the library wants to attach its own user data, it needs to ask you for permission (e.g. store it as a member of your window class wrapper). — Adam Rosenfield
@Adam Rosenffield. Therefor it is safer to use SetProp(,,,) ,GetProp(...) and RemoveProp(...) for this. GWL_USERDATA is frequently used for subclassing — engf-010
Subclassing (in C++'s sense) is what I want - the context data for the window I'm trying to store & access is the class pointer. — inetknght

default default · Accepted Answer · 2010-11-24T19:46:37

if I'm understanding you correctly, you want to avoid one window to catch the messages from another. One way to avoid this is to use the solution proposed in this thread, which keeps track of the windows that is created by you and makes sure that the correct windows gets the messages associated to it by storing the pointer for the caller in the GWL_USERDATA .

// ...
m_hWnd = CreateWindowEx(0,"Classname","Title",WS_OVERLAPPEDWINDOW,
                        CW_USEDEFAULT,CW_USEDEFAULT,
                        320,200,NULL,NULL,hInstance, /*the magic pointer*/ this);

// ...

if(uMsg == WM_CREATE)
{
    // collected here..
    pParent = (CWindow*)((LPCREATESTRUCT)lParam)->lpCreateParams;
    // .. and then stored for later lookup
    SetWindowLongPtr(hWnd,GWL_USERDATA,(LONG_PTR)pParent); 
}
// ...

You can also catch the WM_NCCREATE message, as proposed by Moo-Juice.
And I don't think you should worry about the messages pre-WM_CREATE, because the window isn't even fully initialized at that point. If you need to set text you do that after the call to CreateWindow(Ex), be it user input or a SendMessagecall.

C/C++ - Windows - how to keep track of HWND context data

4 Answers