Question

I’m building a mfc application that will enable the user to draw graphical objects (something like ms paint). But for some reason I’m getting the following linker error:

CElement.obj : error LNK2001: unresolved external symbol “public: virtual void __thiscall CElement::Draw(class CDC *)” (?Draw@CElement@@UAEXPAVCDC@@@Z).

I know it’s something related to the virtual draw function in the CPolygon class. But what hat exactly is causing it?

//CElement.h

class CElement : public CObject
{
 public:
 virtual ~CElement();
 virtual void Draw(CDC* pDC);

};

NOTE: CElement will act as a base class for all other classes like CPolyline and CRectangle. The Draw function is virtual- an example of polymorphism, CElement’s Draw(CDC* pDC) will be overridden by the Draw() functions of the derived classes

class CPolygon : public CElement
{
public:

CPolygon(CPoint mFirstPoint,CPoint mSecondPoint);
~CPolygon(void);
virtual void Draw(CDC* pDC); 

---------------------------------------------------------------------------------------

//CElement.cpp

 #include "CElement.h"

 //constructors for the class

 void CPolygon::Draw(CDC* pDC)
 {
  pDC->MoveTo(mStartPoint);
  pDC->LineTo(mEndPoint);

}

Answer 1

Well as the error message says you have not defined a body for the function

virtual void Draw(CDC* pDC);

either define it or make the class abstract i.e. derived classes must implement it.

virtual void Draw(CDC* pDC) { }

or

virtual void Draw(CDC* pDC) = 0;