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

From:
Ian Collins <ian-news@hotmail.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 20 Aug 2011 09:54:31 +1200
Message-ID:
<9b84coFr4mU3@mid.individual.net>
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.

Code:

//********************************************** test1.h
*******************************//
#include<iostream>
using namespace std;

class CPolygon {


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

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


This isn't defined anywhere.

  protected:
        CRectangle();


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

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


This isn't defined anywhere.

  protected:
    CTriangle();


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);
  ppoly1->printarea();
  ppoly2->printarea();
  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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