global pointers to objects will break program by access violation when accessing object attributes/methods through them

Question

This is code for an OpenGL application i'm making. I have the following code that produces an access violation (notice: i've only posted the code relevant to the problem):

// file: myHeader.h
class MyClass{
    int classAttribute;
  public:
    void myClassFunction();
    bool CreateGLWindow();
    friend LRESULT CALLBACK WndProc(HWND hWnd,UINT uMsg,WPARAM wParam,LPARAM lParam);
    MyClass();
   ~MyClass();
}

//file: myHeader.cpp
bool MyClass::CreateGLWindow(){
    WNDCLASS    wc;
    wc.lpfnWndProc = (WNDPROC) WndProc;
}
void MyClass::myClassFunction(){
    // do stuff
}

MyClass::MyClass(void){
    CreateGLWindow();
}
//file main.cpp
#include myHeader.h

MyClass *p;

LRESULT CALLBACK WndProc(HWND hWnd,UINT uMsg,WPARAM wParam,LPARAM lParam)
{
  if (p->classAttribute){
     p->myClassFunction();
  }
}

int main(){
    MyClass obj;
    p = &obj;
    BOOL done=FALSE;

    while(!done)                                
    {
      if (PeekMessage(&msg,NULL,0,0,PM_REMOVE))// Is There A Message Waiting?
      {
        if (msg.message==WM_QUIT) // Have We Received A Quit Message?
        {
        done=TRUE;
        }
        else    // If Not, Deal With Window Messages
        {
                TranslateMessage(&msg);
                DispatchMessage(&msg);  
        }
      }
      else // If There Are No Messages
      {
    // Draw The Scene.  Watch For ESC Key And Quit Messages From DrawGLScene()
        if (obj.active)                     // Program Active?
        {
          if (obj.keys[VK_ESCAPE])  
          {
        done=TRUE;
          }
          else  // Not Time To Quit, Update Screen
          {
        DrawGLScene();      
        obj.Swap();         // Swap Buffers (Double Buffering)
          }
        }
        if (obj.keys[VK_F1])
        {
          obj.keys[VK_F1]=FALSE;    
          if (!obj.changeScreenMode())
          {
            return 0; // Quit If Window Was Not Created
          }
        }
      }
    }
}
}

What i get is an access violation on the line p->classAttribute and debugging shows that the attributes and methods of p can't be evaluated.

Any help will be greatly appreciated.

You need to add some more code from "//code that will eventually call WndProc". p might not be valid by the time the call happens (in fact it is not valid... access violation), so we need to see what you are doing before that call. — Dennis
There's a missing ; after your class, but I take that's a copy/paste/typing error. :) — Xeo

grayDad grayDad · Accepted Answer · 2011-04-15T10:16:30

The constructor seems to create the window and associate the WndProc before p is assigned (the next line of code)

MyClass obj;
    p = &obj;

You could set MyClass * p = NULL; and then wrap the use of p in

LRESULT CALLBACK WndProc(HWND hWnd,UINT uMsg,WPARAM wParam,LPARAM lParam)
{
  if(p)
  {
     if (p->classAttribute){
        p->myClassFunction();
  }
}

global pointers to objects will break program by access violation when accessing object attributes/methods through them

2 Answers