Thread: Function return value and scope question

Hi there,

I recently just about managed to complete a little challenge I set myself by getting a windows class registered and window created from functions in a DLL instead of from the WinMain function.

However... there's one part of it that is more of a C++ question than a Windows question so I thought I'd ask it here.

It concerns a variable not going out of scope when I thought maybe it would. If I try and declare a HWND variable like this:

Code:

HWND hwnd;

createRunWindow(hInstance, hDll, hwnd);

The compiler will complain (C4700 error code I believe) that the variable is used before being initialised. If I however do this:

Code:

HWND hwnd = nullptr;
or HWND hwnd = 0;

createRunWindow(hInstance, hDll, hwnd);

I get some crazy errors with the linker (probably something windows specific). But if I make the createRunWindow function to do this:

Code:

HWND createRunWindow(HINSTANCE hInstance, HMODULE hDll)
{
    ....some time later

    HWND hwnd;   

    CREATEWINDOW _createWindow = (CREATEWINDOW)GetProcAddress(hDll, "createWindow");
    _createWindow(&hInstance, WindowProc, &hwnd);

    return hwnd;
}

And have the function called in WinMain like this:

Code:

HWND hwnd = createRunWindow(hInstance, hDll);

All works nicely. But here's where it becomes a general C++ question. The WinMain function continues afterwards needing a valid HWND handle for other functions within it to work.

So my question is, given that the function returns a hwnd before exiting, does that mean the line

Code:

HWND hwnd = createRunWindow(hInstance, hDll);

is ok because it's been initialized to something before the temporary variable in the function went out of scope?

Thanks

Thanks very much for a great explanation

HWND is a simple scalar type (I think it's defined as a DWORD) so it's safe to return from a function (as Salem said). In this case the Windows "createWindow" function allocates a window and returns a handle to the window; a handle is kinda-sorta like a pointer.

What's not safe is returning a pointer to an automatic local variable because its lifetime ends when the function returns. On the other hand it is safe to return a pointer to a static local variable since its lifetime is the same as the program, even though it has local scope; scope only limits where you can use an object directly by its name, but lifetime affects how long an object lives and can be used, either directly by its name or indirectly through a pointer or the like.

More good advice thanks very much