TForm: Add a message handler and call inherited - c++builder

Question

Let´s say I need to create a Windows message handler in my C++ Builder cpp file.
I'll write a handler for WM_SIZE like this:
h file:
BEGIN_MESSAGE_MAP
MESSAGE_HANDLER(WM_SIZE, TWMSize, OnWMSize)
END_MESSAGE_MAP(TControl)
void __fastcall OnWMSize(TWMSize &msg);
cpp file:
void __fastcall TForm1::OnWMSize(TWMSize &msg)
{
if (msg.SizeType == SIZE_MAXIMIZED)
Caption = "Maximized";
// This is a Delphi call and won't work in C++:
// inherited;
// Cpp call. Won't compile due to method visibility (private)
// TForm::WMSize(msg);
// Cpp call. Compiles but doesn't get expected behavior
DefaultHandler((void*)&msg);
}
As shown in the example, using Delphi this is pretty straightforward, just call inherited.
But using C++, how do I call the base class in C++ Builder? WMSize method is declared as private in TScrollingWinControl.
In this example, not calling the base class breaks Anchors functionality. Using DefaultHandler won't solve the problem, either.
Am I missing something obvious?

Answer 1

First off, you are passing the wrong class type to END_MESSAGE_MAP(). You need to specify the immediate parent class, which in this case is TForm, not TControl:
BEGIN_MESSAGE_MAP
MESSAGE_HANDLER(WM_SIZE, TWMSize, OnWMSize)
END_MESSAGE_MAP(TForm) // <-- here
Now, that being said, a MESSAGE_MAP is just a fancy way of overriding the virtual Dispatch() method:
#define BEGIN_MESSAGE_MAP virtual void __fastcall Dispatch(void *Message) \
{ \
switch (((PMessage)Message)->Msg) \
{
#define VCL_MESSAGE_HANDLER(msg,type,meth) \
case msg: \
meth(*((type *)Message)); \
break;
// NOTE: ATL defines a MESSAGE_HANDLER macro which conflicts with VCL's macro. The
// VCL macro has been renamed to VCL_MESSAGE_HANDLER. If you are not using ATL,
// MESSAGE_HANDLER is defined as in previous versions of BCB.
//
#if !defined(USING_ATL) && !defined(USING_ATLVCL) && !defined(INC_ATL_HEADERS)
#define MESSAGE_HANDLER VCL_MESSAGE_HANDLER
#endif // ATL_COMPAT
#define END_MESSAGE_MAP(base) default: \
base::Dispatch(Message); \
break; \
} \
}
So, your MESSAGE_MAP resolves to the following code logic:
virtual void __fastcall Dispatch(void *Message)
{
switch (((PMessage)Message)->Msg)
{
case WM_SIZE:
OnWMSize(*((TWMSize *)Message));
break;
default:
TForm::Dispatch(Message);
break;
}
}
As you can see, END_MESSAGE_MAP() simply passes unhandled messages to the base class TForm::Dispatch() method. You must do the same in your message handlers, eg:
void __fastcall TForm1::OnWMSize(TWMSize &msg)
{
if (msg.SizeType == SIZE_MAXIMIZED)
Caption = "Maximized";
TForm::Dispatch(&msg);
}