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

From:

red floyd <no.spam.here@its.invalid>

Newsgroups:

comp.lang.c++

Date:

Fri, 19 Aug 2011 14:47:59 -0700

Message-ID:

<j2mlns$vsu$1@dont-email.me>

On 8/19/2011 2:26 PM, 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:

General:
* Don't bother prefixing your classes with "C". That's an
MFC-ism and is generally unnecessary.
* void param lists are unnecessary. Use void f(), instead of
void f(void).

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

class CPolygon {
  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 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();
  protected:
        CRectangle();
        static CRectangle* rectanglePtr;
        CRectangle(CRectangle const&){};
        CRectangle& operator=(CRectangle&){};
  };

Where are CRectangle() and ~CRectangle() defined?

This should probably not be in a header (though for this
toy example it works)

CRectangle* CRectangle::rectanglePtr = NULL;

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

    static CTriangle* GetInstance()
      { if (!trianglePtr) trianglePtr = new CTriangle(); return
trianglePtr; }
  ~CTriangle();
  protected:
    CTriangle();
    static CTriangle* trianglePtr;
    CTriangle(CTriangle const&){}
    CTriangle& operator=(CTriangle&){};
  };

This should probably not be in a header (though for this
toy example it works)

CTriangle* CTriangle::trianglePtr = NULL;

Similarly, where are CTriangle() and ~CTriangle()
defined()

//******************************************* 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;
}

I don't see what's wrong with the static members,
but you do have to declare those missing constructors
and destructors.