Pure Virtual Method Called terminate - c++

Question

I am following a tutorial on MSDN: http://msdn.microsoft.com/en-us/library/windows/desktop/ff381400(v=vs.85).aspx
Which shows how to make a class to create windows in WINAPI.
I did the exact same thing as them, except that I put the Creation code in the constructor.
When ran, it gives pure virtual method called error and terminates the program with no explanaton.
template<typename T>
class Form
{
public:
Form(std::string Title, std::string Class, /*All other params*/);
private:
static LRESULT __stdcall WindowProcedure(HWND Hwnd, UINT Msg, WPARAM wParam, LPARAM lParam);
protected:
HWND WindowHandle = nullptr;
virtual LRESULT HandleMessages(UINT Msg, WPARAM wParam, LPARAM lParam) = 0;
};
template<typename T>
Form<T>::Form(std::string Title, std::string Class, /*All Other Params*/)
{
WNDCLASSEX WndClass = {/*Filled in with other params*/};
RegisterClassEx(&WndClass);
WindowHandle = CreateWindowEx(/*Other params*/, this);
}
template<typename T>
LRESULT __stdcall Form<T>::WindowProcedure(HWND Hwnd, UINT Msg, WPARAM wParam, LPARAM lParam)
{
T* Data = nullptr;
switch (Msg)
{
case WM_NCCREATE:
{
CREATESTRUCT* pCreate = (CREATESTRUCT*)lParam;
Data = reinterpret_cast<T*>(pCreate->lpCreateParams);
SetWindowLongPtr(Hwnd, GWLP_USERDATA, reinterpret_cast<LONG_PTR>(Data));
break;
}
default:
{
Data = reinterpret_cast<T*>(GetWindowLongPtr(Hwnd, GWLP_USERDATA));
return Data ? Data->HandleMessages(Msg, wParam, lParam) : DefWindowProc(Hwnd, Msg, wParam, lParam);
}
}
return 0;
}
Then my main window:
class MainWindow : public Form<MainWindow>
{
public:
MainWindow(std::string Title, /*Other params*/);
LRESULT HandleMessages(UINT Msg, WPARAM wParam, LPARAM lParam);
HWND WindowHandle() {return Form::WindowHandle;}
};
MainWindow::MainWindow(std::string Title, std::string Class, /*Other params*/) {}
LRESULT MainWindow::HandleMessages(UINT Msg, WPARAM wParam, LPARAM lParam)
{
switch(Msg)
{
case WM_CREATE:
{
}
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(WindowHandle(), Msg, wParam, lParam);
}
return 0;
}
That is all. Why does it give me that error and how can I fix it?

Answer 1

I did the exact same thing as them, except that I put the Creation code in the constructor.
This is why. Calling a virtual method from the constructor doesn't do what you expect - unless you expect it to call the method from the current class in the hierarchy. When a base class constructor executes, the object isn't of derived type yet. And if you call a virtual method which is pure in the current class from the constructor of that class, you'll get that error.