Conference 7.286::visualc

1098.0. "Compiler error (VC++4.2) in tutorial application: HELP!" by GRIFUN::BENVI () Wed Mar 26 1997 03:27

HI all,
	while trying to learn MFC prpgramming I followed the scribble example supplied in
VC++4.2 installation (books online/Visual C++ books/Tutorials/...)
I strictly followed the indication on the documenation copying and pasting the code
supplied in the sections of the docs.
When I tried the compilation I received the following errors:

Compiling...
scribbleDoc.cpp
C:\MSDEV\MFC\include\afxtempl.h(1561) : 
error C2664: 'struct __POSITION *CObList::AddHead(class CObject *)' : 
cannot convert parameter 1 from 'class CStroke *' to 'class CObject *' (new behavior; please see help)

C:\MSDEV\MFC\include\afxtempl.h(1563) : 
error C2664: 'struct __POSITION *CObList::AddTail(class CObject *)' : 
cannot convert parameter 1 from 'class CStroke *' to 'class CObject *' (new behavior; please see help)

C:\MSDEV\MFC\include\afxtempl.h(1587) : 
error C2664: 'SetAt' : cannot convert parameter 2 from 'class CStroke *' to 'class CObject *' 
(new behavior; please see help)

scribbleView.cpp
C:\MSDEV\MFC\include\afxtempl.h(1561) : 
error C2664: 'struct __POSITION *CObList::AddHead(class CObject *)' : 
cannot convert parameter 1 from 'class CStroke *' to 'class CObject *' (new behavior; please see help)

C:\MSDEV\MFC\include\afxtempl.h(1563) : 
error C2664: 'struct __POSITION *CObList::AddTail(class CObject *)' : 
cannot convert parameter 1 from 'class CStroke *' to 'class CObject *' (new behavior; please see help)

C:\MSDEV\MFC\include\afxtempl.h(1587) : 
error C2664: 'SetAt' : cannot convert parameter 2 from 'class CStroke *' to 'class CObject *' 
(new behavior; please see help)

scribble.cpp
C:\MSDEV\MFC\include\afxtempl.h(1561) : 
error C2664: 'struct __POSITION *CObList::AddHead(class CObject *)' : 
cannot convert parameter 1 from 'class CStroke *' to 'class CObject *' (new behavior; please see help)

C:\MSDEV\MFC\include\afxtempl.h(1563) : 
error C2664: 'struct __POSITION *CObList::AddTail(class CObject *)' : 
cannot convert parameter 1 from 'class CStroke *' to 'class CObject *' (new behavior; please see help)
C:\MSDEV\MFC\include\afxtempl.h(1587) : 
error C2664: 'SetAt' : cannot convert parameter 2 from 'class CStroke *' to 'class CObject *' 
(new behavior; please see help)

Generating Code...
Error executing cl.exe.
Creating browse info file...
scribble.exe - 9 error(s), 0 warning(s)

The error refer to the declaration of template calss CTyped PtrList (I didn't modify this)

// CTypedPtrList<BASE_CLASS, TYPE>

template<class BASE_CLASS, class TYPE>
class CTypedPtrList : public BASE_CLASS
{
public:
// Construction
	CTypedPtrList(int nBlockSize = 10)
		: BASE_CLASS(nBlockSize) { }
................
	POSITION AddHead(TYPE newElement)
		{ return BASE_CLASS::AddHead(newElement); }
	POSITION AddTail(TYPE newElement)
		{ return BASE_CLASS::AddTail(newElement); }
...............

The example uses CTypedPtrList in a couple of places:
In the interfacce file of class CScribbleDoc
 
- class CScribbleDoc : public CDocument
{
protected: // create from serialization only
	void InitDocument();
	CPen m_penCur;
	UINT m_nPenWidth;
	CScribbleDoc();
	DECLARE_DYNCREATE(CScribbleDoc)

// Attributes
public:
	CStroke* NewStroke();

// Operations
public:
	CTypedPtrList<CObList, CStroke*> m_strokeList; <-----
	CPen* GetCurrentPen() { return &m_penCur; }
...........
}

- and in implementation file of class CScribbleView (the view associated to document)

void CScribbleView::OnDraw(CDC* pDC)
{
	CScribbleDoc* pDoc = GetDocument();
	ASSERT_VALID(pDoc);

	// TODO: add draw code for native data here

	// The view delegates the drawing of individual strokes to
	// CStroke::DrawStroke( ).
	
	CTypedPtrList<CObList, CStroke*>& strokeList =  <--------
			pDoc->m_strokeList;
	POSITION pos = strokeList.GetHeadPosition( );
	while (pos != NULL)
	{
		CStroke* pStroke = strokeList.GetNext(pos);
		pStroke->DrawStroke( pDC );
	}

}
........

Class CStroke is declared as follows:

class CStroke : public CObject
{
public:
	CStroke( UINT nPenWidth );

protected:
	CStroke( );
	DECLARE_SERIAL( CStroke )

// Attributes
protected:
	UINT   m_nPenWidth;  // One width applies to entire stroke
public:
	CArray<CPoint, CPoint>   m_pointArray;  // Series of connected						
						// points

// Operations
public:
	BOOL DrawStroke( CDC* pDC );

public:
	virtual void Serialize( CArchive& ar );
};

I checked the artcles posted in books online\README for Microsoft Visual C++ 4.2/C/C++ LAnguage\...
and I found a reference to the error I got as a more reobust implementation of type checking 
in "User Defined Conversion Selected by 'this' pointer type", but the examples posted there
don't seem to be related to my situation...

Can Anybody help me to understand the error and to eliminate it?
Please help a novice programmer!!!!

Thanks
	Angelo

The completed solution is available online, on the Visual C++
CD, and it builds correctly.

If you look at the \SAMPLES directory on the VC++ CD,
the completed Scrible application is in 
\SAMPLES\MFC\TUTORIAL\SCRIBBLE\STEPn (n = 1 to 8,
corresponding to various additions to the basic scribble
functionality.) According to the VC++ online docs, the
end of the Scribble online tutorial (when you've added
context-sensitive help) corresponds to Step 6 of the
supplied Scribble samples. You could use the WinDiff utility
to compare the contents of your project directory with
the \SAMPLES\MFC\TUTORIAL\SCRIBBLE\STEP6 project directory.