Handling windows messages from a subclass - c++

Question

I have custom window encapsulated in a class with a static WndProc function to handle messages generated by it. Now I have a child class which implements some of the features in message handling proc differently then the parent class.
For example in the code below, what happens in WM_LBUTTONDOWN in the child class is different then what happens in the parent class.
I thought about polymorphism, but I think it won't work since the ::SetWindowLongPtr() is called from the parent class and "this" pointer passed to belongs to the parent class, correct me if I am wrong.
And if I am wrong and polymorphism will work in this case then too there are some messages which are not handled by the parent class and should be handled in the child class and putting an empty virtual function in the parent class just for that doesn't seems clean, beside its hard to put an empty virtual function for each and every message that window produce just for the sake of if in future it will be used.
There going to be several such child classes each one of them behaving differently for some messages but not all.
So, how should I go about doing it.
parent.cpp
parent::parent()
{
WNDCLASSEX wincl;
wincl.hInstance = hInstance;
wincl.lpszClassName = "parent";
wincl.lpfnWndProc = WndProc;
wincl.style = CS_BYTEALIGNWINDOW;
wincl.cbSize = sizeof (WNDCLASSEX);
wincl.hIcon = 0;
wincl.hIconSm = 0;
wincl.hCursor = ::LoadCursor (NULL, IDC_ARROW);
wincl.lpszMenuName = NULL;
wincl.cbClsExtra = 0;
wincl.cbWndExtra = 4;
wincl.hbrBackground = ::CreateSolidBrush( backgroundColor );
::RegisterClassEx ( &wincl );
hwnd = ::CreateWindowEx ( 0, "parent", txt.c_str(), WS_CLIPCHILDREN | WS_CLIPSIBLINGS | WS_CHILD, x, y, width, height, parent, 0, hInstance, 0 ) ;
::SetWindowLongPtr( hwnd , GWLP_USERDATA , ( LONG ) this ) ;
}
LRESULT CALLBACK parent::WndProc ( HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam )
{
view::parent* panel = reinterpret_cast < view::parent* > ( ::GetWindowLongPtr ( hwnd , GWLP_USERDATA ) );
switch (message)
{
case WM_CREATE:
::SendMessage ( hwnd, WM_SETFONT, ( WPARAM ) panel->hFont, ( LPARAM ) true );
break ;
case WM_COMMAND:
return panel->command ( message, wParam, lParam );
break ;
case WM_LBUTTONDOWN:
return panel->lButton ( message, wParam, lParam );
break;
case WM_RBUTTONDOWN:
return panel->rButton ( message, wParam, lParam );
break;
case WM_ERASEBKGND:
return 1;
break;
case WM_PAINT:
return panel->paint ( );
break ;
default:
return ::DefWindowProc (hwnd, message, wParam, lParam);
}
return 0 ;
};
Thanks.

Answer 1

Try something like this:
class parent
{
private:
// ...
static LRESULT CALLBACK WndProcCallback(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam);
protected:
HWND m_hwnd;
// ...
virtual LRESULT WndProc(UINT message, WPARAM wParam, LPARAM lParam);
virtual LRESULT DefWndProc(UINT message, WPARAM wParam, LPARAM lParam);
virtual LRESULT command(WPARAM wParam, LPARAM lParam);
virtual LRESULT lButtonDown(WPARAM wParam, LPARAM lParam);
virtual LRESULT rButtonDown(WPARAM wParam, LPARAM lParam);
virtual LRESULT paint();
// ...
public:
parent();
virtual ~parent();
// ...
};
parent::parent()
{
WNDCLASSEX wincl = {0};
wincl.hInstance = hInstance;
wincl.lpszClassName = "parent";
wincl.lpfnWndProc = WndProcCallback;
wincl.style = CS_BYTEALIGNWINDOW;
wincl.cbSize = sizeof(WNDCLASSEX);
wincl.hIcon = 0;
wincl.hIconSm = 0;
wincl.hCursor = ::LoadCursor(NULL, IDC_ARROW);
wincl.lpszMenuName = NULL;
wincl.cbClsExtra = 0;
wincl.cbWndExtra = 4;
wincl.hbrBackground = ::CreateSolidBrush(backgroundColor);
::RegisterClassEx(&wincl);
m_hwnd = NULL;
::CreateWindowEx(0, "parent", txt.c_str(), WS_CLIPCHILDREN | WS_CLIPSIBLINGS | WS_CHILD, x, y, width, height, parent, 0, hInstance, this);
}
parent::~parent()
{
if (m_hwnd)
DestroyWindow(m_hwnd);
}
LRESULT CALLBACK parent::WndProcCallback(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
parent* panel;
if (message == WM_CREATE)
{
CREATESTRUCT *cs = reinterpret_cast<CREATESTRUCT*>(lParam);
panel = static_cast<parent*>(cs->lpCreateParams);
panel->m_hwnd = hwnd;
::SetWindowLongPtr(hwnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(panel));
}
else
panel = reinterpret_cast<parent*>(::GetWindowLongPtr(hwnd, GWLP_USERDATA));
if (panel)
return panel->WndProc(message, wParam, lParam);
return ::DefWindowProc(hwnd, message, wParam, lParam);
}
LRESULT parent::WndProc(UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_CREATE:
::SendMessage(m_hwnd, WM_SETFONT, (WPARAM) hFont, TRUE);
break ;
case WM_COMMAND:
return command(wParam, lParam);
break ;
case WM_LBUTTONDOWN:
return lButtonDown(wParam, lParam);
break;
case WM_RBUTTONDOWN:
return rButtonDown(wParam, lParam);
break;
case WM_ERASEBKGND:
return 1;
break;
case WM_PAINT:
return paint();
break;
}
return DefWndProc(message, wParam, lParam);
}
LRESULT parent::DefWndProc(UINT message, WPARAM wParam, LPARAM lParam)
{
return ::DefWindowProc(m_hwnd, message, wParam, lParam);
}
LRESULT parent::command(WPARAM wParam, LPARAM lParam)
{
return DefWndProc(WM_COMMAND, wParam, lParam);
}
LRESULT parent::lButtonDown(WPARAM wParam, LPARAM lParam)
{
return DefWndProc(WM_LBUTTONDOWN, wParam, lParam);
}
LRESULT parent::rButtonDown(WPARAM wParam, LPARAM lParam)
{
return DefWndProc(WM_RBUTTONDOWN, wParam, lParam);
}
LRESULT parent::paint()
{
return 0;
}
Not only does this embrace polymorphism and encapsulation, but making WndProc() itself virtual allows derived classes to override behavior for any received message (well, any message received after WM_CREATE that is), especially messages the parent has no concept of:
class child : public parent
{
protected:
LRESULT WndProc(UINT message, WPARAM wParam, LPARAM lParam);
};
LRESULT child::WndProc(UINT message, WPARAM wParam, LPARAM lParam)
{
if (message == WM_SOMETHING)
{
// ...
return 0;
}
return parent::WndProc(message, wParam, lParam);
}