Re: Linker Error: Abstract base and 2 derived classes in singleton pattern

Ian Collins <>
Sat, 20 Aug 2011 09:54:31 +1200
On 08/20/11 09:26 AM, Vinesh S wrote:

Hello all,

This is my first message to the group.

I have a question where i derive 2 child classes which are of
singleton pattern from an abstract base class.


//********************************************** test1.h
using namespace std;

class CPolygon {

Why not just class Polygon? The 'C' is just noise.

    int width, height;
    virtual ~CPolygon(){};
    void set_values (int a, int b)
      { width=a; height=b; }
    virtual int area (void) = 0;

(void) is a Cism best not used in C++ code.

    void printarea (void)
      { cout<< this->area()<< endl; }


class CRectangle: public CPolygon {
    int area (void)
      { return (width * height); }
    static CRectangle* GetInstance()
      { if (!rectanglePtr) rectanglePtr = new CRectangle(); return
rectanglePtr; }

This isn't defined anywhere.


This isn't defined anywhere.

        static CRectangle* rectanglePtr;
        CRectangle(CRectangle const&){};
        CRectangle& operator=(CRectangle&){};

This shouldn't compile, missing return (also superfluous semi-colon).


CRectangle* CRectangle::rectanglePtr = NULL;

This shouldn't be in a header, static members must be defined once and
only once. Put the definition in a cpp file.

class CTriangle: public CPolygon {
    int area (void)
      { return (width * height / 2); }

    static CTriangle* GetInstance()
      { if (!trianglePtr) trianglePtr = new CTriangle(); return
trianglePtr; }

This isn't defined anywhere.


This isn't defined anywhere.

    static CTriangle* trianglePtr;
    CTriangle(CTriangle const&){}
    CTriangle& operator=(CTriangle&){};

This shouldn't compile, missing return.


CTriangle* CTriangle::trianglePtr = NULL;

//******************************************* test1.cpp
#include "test1.h"

int main (void)
  CPolygon *ppoly1 = CRectangle::GetInstance();
  CTriangle *ppoly2 = CTriangle::GetInstance();
  ppoly1->set_values (4,5);
  ppoly2->set_values (4,5);
  delete ppoly1;
  delete ppoly2;
  return 0;

Please let me know as to what mistake am i doing .

Try the fixes I suggested and it should link.

Ian Collins

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