Question

I was playing with visual studio’s windows forms and the example code had this:

INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
    /*code cut*/
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;
    break;

While my other handler function looks like this:

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
    /*code cut*/
    return 0; // no magical casting or anything, just plain int

I have a clue that in WndProc() when i do a return 0; it means the message wont get handled by the default message handler? but if you do return 1; it would handle with default handler?

But what is the point of return (INT_PTR)TRUE; ? and is it safe to use plain return 0; style there? i tried to compile and it works with just plain integer values too.

Also, im not sure when i should use which of the values, the example code had this:

EndDialog(hDlg, LOWORD(wParam));
return (INT_PTR)TRUE;

But what is the point of the return (INT_PTR)TRUE; there? I changed it to FALSE but couldnt see any difference in functionality.

So, im not sure what this is doing, could anyone clear my mind on this, when should i use return 1 and when should i use return 0, and when should i use something else (if i should) ?

Answer 1

The first piece of code is a DialogProc — quoting the relevant docs:

Return Value
Type: INT_PTR
Typically, the dialog box procedure should return TRUE if it processed the message, and FALSE if it did not. If the dialog box procedure returns FALSE, the dialog manager performs the default dialog operation in response to the message.

The second piece of code is a WindowProc — quoting the relevant docs:

Return Value
Type: LRESULT
The return value is the result of the message processing and depends on the message sent.

So, that /*code cut*/ part is critically relevant, because which return value you want depends entirely on which message is being handled.