Question

Ok this is a really weird problem. I wanna start off by saying that I’m not a beginner in c++ and I’m certainly not advanced. I’m somewhere in the middle. What I’m trying to do is make a C++ OOP wrapper library (dll) of the Win32 API. Here are the classes of my library. I compiled it with Mingw using the command:

g++ -shared -o bin\win32oop.dll src\Application.cpp src\Form\Form.cpp -Wall

src\Application.h:

#ifndef WOOP_APPLICATION_H_
#define WOOP_APPLICATION_H_

namespace Woop
{
 class Application
 {
 public:
  bool Init(void);
  virtual bool OnInit(void);
 };
}

#endif // WOOP_APPLICATION_H_

src\Application.cpp

#include <windows.h>
#include "Application.h"
#include "Form\Form.h"

LRESULT CALLBACK WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);

namespace Woop
{
 bool Application::Init(void)
 {
  WNDCLASSEX wc;

  wc.cbSize        = sizeof(WNDCLASSEX);
  wc.style         = 0;
  wc.lpfnWndProc   = WndProc;
  wc.cbClsExtra    = 0;
  wc.cbWndExtra    = 0;
  wc.hInstance     = GetModuleHandle(NULL);
  wc.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
  wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
  wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
  wc.lpszMenuName  = NULL;
  wc.lpszClassName = "woop";
  wc.hIconSm       = LoadIcon(NULL, IDI_APPLICATION);

  if(RegisterClassEx(&wc) == 0)
  {
   MessageBox(NULL, "Window Registration Failed!", "Error!", MB_ICONEXCLAMATION | MB_OK);
   return false;
  }

  this->OnInit();

  return true;
 }

 bool Application::OnInit(void)
 {
  return true;
 }
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    Woop::Form *wnd = 0;

    if (uMsg == WM_NCCREATE) 
 {   
        SetWindowLong (hwnd, GWL_USERDATA, long((LPCREATESTRUCT(lParam))->lpCreateParams));
    }

 wnd = (Woop::Form *)(GetWindowLong (hwnd, GWL_USERDATA));

    if (wnd) return wnd->WndProc(hwnd, uMsg, wParam, lParam);

    return ::DefWindowProc (hwnd, uMsg, wParam, lParam);
}

src\Form\Form.h

#ifndef WOOP_FORM_FORM_H_
#define WOOP_FORM_FORM_H_

namespace Woop
{
 class Form
 {
 public:
  bool Show(void);
  virtual LRESULT WndProc(HWND, UINT, WPARAM, LPARAM);
 protected:
  HWND _handle;
 };
}

#endif // WOOP_FORM_FORM_H_

src\Form\Form.cpp

#include <windows.h>
#include "Form.h"

namespace Woop
{
 bool Form::Show(void)
 {
  _handle = CreateWindowEx(WS_EX_CLIENTEDGE, "woop", "", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, 240, 120, NULL, NULL, GetModuleHandle(NULL), this);

  if(_handle == NULL)
  {
   MessageBox(NULL, "Window Creation Failed!", "Error!", MB_ICONEXCLAMATION | MB_OK);
   return false;
  }

  ShowWindow(_handle, SW_SHOWNORMAL);

  return true;
 }

 LRESULT Form::WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
 {
  switch(uMsg)
  {
   case WM_DESTROY:
    PostQuitMessage(0);
   break;            
  }
  return DefWindowProc(hwnd, uMsg, wParam, lParam);
 }
}

Here is a the program that I’m testing the library with:

class SampleApp : public Woop::Application
{
 bool OnInit(void)
        {
         Form form;
         form.Show();

         return true;
        }
};

INT APIENTRY WinMain(HINSTANCE, HINSTANCE, LPSTR, INT)
{
 SampleApp application;
 if(application.Init() == false) return 0;

 MSG Msg;
 while(GetMessage(&Msg, NULL, 0, 0) > 0)
    {
        TranslateMessage(&Msg);
        DispatchMessage(&Msg);
    }

 return 0;
}

Alright, now the problem. Do you see that virtual Window Procedure in the Form class? If I remove the virtual from the declaration, the program compiles and runs fine. But when I add it back, it crashes. The infamous “Don’t Send” dialog comes up. I’m not sure when it crashes, i’ll try to figure that out using MessageBox() (lol, it’s what I get for not learning how to debug with gdb). I’m trying to make it so that I can make a class such as LoginForm and derive from Form and override the Window Procedure. I hope I explained the problem well enough :D. This might be a compiler bug or my stupidity :P. Anyways, thanks in advance.

Answer 1

The problem is here:

bool OnInit(void) 
{ 
     Form form; 
     form.Show(); 

     return true; 
}

The form object is destroyed when this method is returned.
So the this pointer that you stored when you call Show() is no longer valid.

  _handle = CreateWindowEx(WS_EX_CLIENTEDGE, "woop", "", WS_OVERLAPPEDWINDOW,
                           CW_USEDEFAULT, CW_USEDEFAULT, 240, 120, NULL, NULL,
                           GetModuleHandle(NULL), 
  /* Here ----> */         this
                          ); 

When you try and do the dispatch it is getting reallyed screwed up because it is using the this pointer to work out the address of the virtual function to call.

The reason that it crasshes with virtual and not when you take virtual away is that virtual method address is calculated at runtime while the normal method address is planted at compile time.

When calculating the address of the virtual method the this pointer is dereferenced in some way (which in this case leads to UB) but because the object is been destroyed the data at that address has proably been re-used so the address you get for the function is some random junk and calling this will never be good.

A simple solution is to make the form part of the application object.
Thus its lifetime is the same as the application:

class SampleApp : public Woop::Application 
{ 
    Form form;

    bool OnInit(void) 
    { 
        form.Show(); 

        return true; 
    } 
};