为什么用指针调用GetWindowRect会导致异常，但地址却没有

Question

I have a utility function in a dll to center my form on the host applications screen. I'm using RAD Studio XE2. I have to do it manually because the host application is non-VCL and the form placement parameters of the TForm don't work properly. The code below works. Both functions are declared static, and I've previously set the Application handle property to the host app.

   void MyClass::GetAppCenter(POINT * pos) {
      RECT Rect;
      GetWindowRect(Application->Handle, &Rect);
      pos->x = (Rect.left + Rect.right) / 2;
      pos->y = (Rect.top + Rect.bottom) / 2;
   }

   void MyClass::PlaceForm(TForm * f) {
      POINT pos;
      GetAppCenter(&pos);
      for (int i = 0; i < Screen->MonitorCount; i++) {
         TRect r = Screen->Monitors[i]->WorkareaRect;
         if (r.Contains(pos)) {
            f->Left = (r.Left + r.Right) / 2 - f->Width / 2;
            f->Top = (r.Top + r.Bottom) / 2 - f->Height / 2;
            return;
         }
      }
   }

My initial GetAppCenter code used a Rect * instead and returned the correct value, but threw an Access Violation exception when I set f->Left. Can anyone explain why?

   // original version
   void OasisUtils::GetOasisCenter(POINT * pos) {
      RECT *Rect;
      GetWindowRect(Application->Handle, Rect);
      pos->x = (Rect->left + Rect->right) / 2;
      pos->y = (Rect->top + Rect->bottom) / 2;
      delete Rect; // tried with and without this
   }

Answer 1

  RECT *Rect;
  GetWindowRect(Application->Handle, Rect);
  //Rect->left 

This is incorrect. GetWindowRect needs a valid RECT* argument so that it fills memory pointed to by this pointer. You are instead passing an uninitialized pointer expecting that some magic will make it valid. Instead you are getting access violation. You need:

  RECT Rect;
  GetWindowRect(Application->Handle, &Rect); // <<--- Note &
  //Rect.left 

Answer 2

GetWindowRect expects the caller to own the rectangle structure.

In your original version, your *Rect does not point to any valid memory. So when you try to use it, you're accessing some random block of memory that you don't own. The operating system denies this. I am just surprised that the call to GetWindowRect doesn't cause the crash.

On the other hand, your updated version declares RECT Rect , which allocates memory on the stack. That memory is automatically allocated when your function is called, and is cleaned up when your function finishes.

To clarify, this problem doesn't have to do with differences between pointer and reference. The problem is exclusively due to memory allocation / ownership.