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Visual C++ "Access violation accessing location..."

ahcfan (139)
I have these two functions listed below. DoEvent() gets called from a window procedure whenever some event occurs. SetEvent() sets an element of the m_Events array a pointer to a Command object to handle the event. The array is zeroed in the constructor.

The error occurs on line 16 in the code in DoEvent(). It's bugging me out because I have absolutely no idea why the array is suddenly unreadable. When I call SetEvent() it works just fine, no problem.

When I step through the debugger in SetEvent i see that each element begins as 0 and the element I set is set properly. However, when I step through DoEvent() all elements in m_Events show "?????" "Unable to read memory".

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uw::Command* uw::WindowBase::SetEvent(uw::WindowEventsEnum event, uw::Command* command)
{
	if (event < 0 || event >= FINAL_EVENT_ENTRY)
		return NULL;

	Command* ret = m_Events[event];
	m_Events[event] = command;
	return ret;
}

BOOL uw::WindowBase::DoEvent(uw::WindowEventsEnum event, LPVOID pdata)
{
	if (event < 0 || event >= FINAL_EVENT_ENTRY)
		return FALSE;

	if (!m_Events[event])
		return FALSE;

	return m_Events[event]->Do(pdata);
}


Here is the definition of WindowBase:
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class WindowBase
{
public:
	WindowBase();
	virtual ~WindowBase();

	BOOL Create(LPCWSTR caption, LPCWSTR wndclass, DWORD styles, DWORD exstyles = 0, HWND parent = NULL);
	
	Command* SetEvent(WindowEventsEnum event, Command* command);
	BOOL DoEvent(WindowEventsEnum event, LPVOID pdata);

protected:
	HWND m_Window;
	HWND m_Parent;
	HDC m_WindowDC;
	RECT m_WindowRect;
	RECT m_ClientRect;

	Command* m_Events[FINAL_EVENT_ENTRY];

	WNDPROC m_DefProc;

	static LRESULT CALLBACK s_SubProc(HWND, UINT, WPARAM, LPARAM);

};


Please let me know if you need more info.
Last edited on
ahcfan (139)
For the record here's the WindowEventsEnum enum and the Command class:
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enum WindowEventsEnum {
	EVENT_ONDESTROY,		// X
	EVENT_ONERASE,			// X
	EVENT_ONPAINT,			// X
	EVENT_ONMOVE,			// X
	EVENT_ONSIZE,			// X
	EVENT_ONMOUSEPRESS,		// X
	EVENT_ONMOUSERELEASE,	// X
	EVENT_ONMOUSEMOVE,		// X
	EVENT_ONMOUSEWHEEL,		// X
	EVENT_ONKEYPRESS,		// X
	EVENT_ONKEYRELEASE,		// X
	EVENT_ONKEYCHAR,		// X

	FINAL_EVENT_ENTRY
};

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class Command
{
public:
	virtual ~Command() {}

	virtual BOOL Do(LPVOID pdata) = 0;
	virtual BOOL Undo() { return FALSE; }

};


What I really don't get is that everything was working swimmingly until I implemented those last three key events, and then this error started happening.

I think the error comes from the window procedure but I don't see where.

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LRESULT CALLBACK uw::WindowBase::s_SubProc(HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam)
{
	EventStruct evt = {(WindowBase*)GetWindowLongPtr(hwnd, GWLP_USERDATA)};

	switch (msg)
	{
	case WM_DESTROY:
		if (evt.WinObj->DoEvent(EVENT_ONDESTROY, &evt))
			return 0;

		break;

	case WM_MOVE:
	case WM_MOVING:
		GetWindowRect(evt.WinObj->m_Window, &evt.WinObj->m_WindowRect);
		GetClientRect(evt.WinObj->m_Window, &evt.WinObj->m_ClientRect);
		
		if (evt.WinObj->DoEvent(EVENT_ONMOVE, &evt))
			return 0;

		break;

	case WM_SIZE:
	case WM_SIZING:
		GetWindowRect(evt.WinObj->m_Window, &evt.WinObj->m_WindowRect);
		GetClientRect(evt.WinObj->m_Window, &evt.WinObj->m_ClientRect);

		if (evt.WinObj->DoEvent(EVENT_ONSIZE, &evt))
			return 0;

		break;


	case WM_LBUTTONDOWN:
	case WM_MBUTTONDOWN:
	case WM_RBUTTONDOWN:

		evt.MouseEvent.CursorPos.x = GET_X_LPARAM(lparam);
		evt.MouseEvent.CursorPos.y = GET_Y_LPARAM(lparam);

		if (msg == WM_LBUTTONDOWN)
			evt.MouseEvent.Button = VK_LBUTTON;
		else if (msg == WM_MBUTTONDOWN)
			evt.MouseEvent.Button = VK_MBUTTON;
		else if (msg == WM_RBUTTONDOWN)
			evt.MouseEvent.Button = VK_RBUTTON;

		if (evt.WinObj->DoEvent(EVENT_ONMOUSEPRESS, &evt))
			return 0;

		break;

	case WM_LBUTTONUP:
	case WM_MBUTTONUP:
	case WM_RBUTTONUP:

		evt.MouseEvent.CursorPos.x = GET_X_LPARAM(lparam);
		evt.MouseEvent.CursorPos.y = GET_Y_LPARAM(lparam);

		if (msg == WM_LBUTTONUP)
			evt.MouseEvent.Button = VK_LBUTTON;
		else if (msg == WM_MBUTTONUP)
			evt.MouseEvent.Button = VK_MBUTTON;
		else if (msg == WM_RBUTTONUP)
			evt.MouseEvent.Button = VK_RBUTTON;

		if (evt.WinObj->DoEvent(EVENT_ONMOUSERELEASE, &evt))
			return 0;

		break;

	case WM_MOUSEMOVE:
		evt.MouseEvent.CursorPos.x = GET_X_LPARAM(lparam);
		evt.MouseEvent.CursorPos.y = GET_Y_LPARAM(lparam);

		if (evt.WinObj->DoEvent(EVENT_ONMOUSEMOVE, &evt))
			return 0;

		break;

	case WM_MOUSEWHEEL:
		evt.MouseEvent.WheelDelta = GET_WHEEL_DELTA_WPARAM(wparam);
		evt.MouseEvent.CursorPos.x = GET_X_LPARAM(lparam);
		evt.MouseEvent.CursorPos.y = GET_Y_LPARAM(lparam);

		if (evt.WinObj->DoEvent(EVENT_ONMOUSEWHEEL, &evt))
			return 0;

		break;

	case WM_KEYDOWN:
		evt.KeyEvent.Key = wparam;

		if (evt.WinObj->DoEvent(EVENT_ONKEYPRESS, &evt))
			return 0;

		break;

	case WM_KEYUP:
		evt.KeyEvent.Key = wparam;

		if (evt.WinObj->DoEvent(EVENT_ONKEYRELEASE, &evt))
			return 0;

		break;

	case WM_CHAR:
		evt.KeyEvent.Char = (WCHAR)wparam;

		if (evt.WinObj->DoEvent(EVENT_ONKEYCHAR, &evt))
			return 0;

		break;

	case WM_ERASEBKGND:
		if (evt.WinObj->DoEvent(EVENT_ONERASE, &evt))
			return TRUE;

		break;

	case WM_PAINT:
		if (evt.WinObj->DoEvent(EVENT_ONPAINT, &evt))
			return 0;

		break;

	case WM_SETTEXT:
		// (LPWSTR)lparam
		break;
	}

	return evt.WinObj->m_DefProc(hwnd, msg, wparam, lparam);
}
ahcfan (139)
Wanna know something funny? I struggled with this error for hours only to find the solution almost instantly after posting this topic.

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	struct MouseEventStruct {
		WindowBase* WinObj;
		INT Button;
		INT WheelDelta;
		POINT CursorPos;
	};

	struct KeyEventStruct {            // <---- was missing WindowBase* WinObj
		INT Key;
		WCHAR Char;
	};

	union EventStruct {
		WindowBase* WinObj;
		MouseEventStruct MouseEvent;
		KeyEventStruct KeyEvent;
	};
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