I am trying to get this program to compile but i’m getting all sorts

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Asked: June 12, 20262026-06-12T11:47:02+00:00 2026-06-12T11:47:02+00:00

I am trying to get this program to compile but i’m getting all sorts

0

I am trying to get this program to compile but i’m getting all sorts of weird errors
its saying that i’m making and instants of an abstract base class. but i believe it’s only saying that because when i declare the derived class that tries to make an instances of the base class which is abstract.

Im getting multiple declaration for some methods as well.

Thank you so much for your help. im so lost.

the reason i included everything is because i really don’t know what is causing the problems. The error msgs are at the bottom if you want to specifically see what files i’m getting the errors in.

//main.cpp
#include <iostream>
#include "Polygon.h"
using namespace std;
 int main() {
    Polygon rect("Rectangle", 4);
    rect(0) = Point(1, 1);
    rect(1) = Point(2, 1);
    rect(2) = Point(2, 4);
    rect(3) = Point(1, 4);

    cout << rect << endl;

    return 0;
 }
//point.cpp
#include<ostream>
#include"point.h"
using namespace std;

Point::Point()
{
  set_x(0);
  set_y(0);
}

Point::Point(double varX, double varY)
{
  set_x(varX);
  set_y(varY);
}

void Point::set_x(double var)
{
  x=var;
}

void Point::set_y(double var)
{
  y=var;
}

ostream& Point::display(ostream& os) const
{
 os << '(' << x << ',' << y << ')' <<endl;
 return os;
}

friend ostream& operator<<(ostream& os,Point& a)
{
  a.display(os);
}

double Point::xget()
{
 return x;
}
double Point::yget()
{
  return y;
}

Point& operator+(Point& b)
{
  double tempX=x()+b.x();
  double tempY=y()+b.y();
  Point c(tempX,tempY);
  return *c;
}

Point& operator/(Point& o,int value)
{
  double tempX=o.x()/value;
  double tempY=o.y()/value;
  Point c(tempX,tempY);
  return *c;
}
//point.h
#ifndef _point_h
#define _point_h
class Point : public Showable
{
double x;
double y;
public:
Point();
Point(double,double);
ostream& display(ostream&) const;
double xget();
double yget();
void set_x(double);
void set_y(double);
friend ostream& operator<<(ostream&,Point&);
};

Point& operator+(Point&);
Point& operator/(Point&,int);
#endif
//polygon.cpp
//abstract Base Classes
#include <iostream>
#include "Polygon.h"
#include "point.h"
#include "shape.h"
using namespace std;

Polygon::Polygon(char* str , int V):Shape(str)
{
  points=V;
  Point* vertex= new Point[V];
}

Point& Polygon::operator()(int i)
{
  return vertex[i];
}

int Polygon::pointsNum()
{
 return points;
}

Polygon& Polygon::operator=(Polygon& b)
{
  points=b.pointsNum();
  if(vertex)
  delete[] vertex;

  if(this!=&b && b!=NULL)
  {
    vertex=new Point[b.pointNum()];
    for(int i=0;i<b.pointNum();i++)
    {
      vertex[i]=b.vertex[i];
    }
  }
  else
  {
    vertex=NULL;
  }
  return *this;
}
ostream& Polygon::display(ostream& os) const
{
  os << (Shape)(*this) <<" Centroid " << centroid();
}

friend ostream& operator<<(ostream& os,Polygon& a)
{
  a.display(os);
  return os;
}


Point& Polygon::centroid() const
{
  int tempX=0,tempY=0;
  for(int i=0;i<points;i++)
  {
    tempX+= vertex[i].xget();
  }
  for(int i=0;i<points;i++)
  {
    tempY+= vertex[i].yget();
  }
  Point c(tempX/points,tempY/points);
  return *c;
}
//polygon.h
#ifndef _polygon_h
#define _polygon_h
#include "shape.h"
#include "point.h"

class Polygon : public Shape
{
  int points;
  Point* vertex;
  public:
  Polygon(char* , int);
  Point& operator()(int);
  Polygon& Polygon::operator=(Polygon& b);
  int pointsNum();
  ostream& display(ostream& os) const;
  Point& centroid() const;
  friend ostream& operator<<(ostream&,Polygon&);
};


#endif
//shape.cpp
#include<cstring>
#include<ostream>
#include "shape.h"
using namespace std;
Shape::Shape()
{
  name[0]='\0';
}
Shape::Shape(char* str)
{
  strcpy(name,str);
}

ostream& Shape::display(ostream& os) const
{
  os << name;
  return os;
}

char* Shape::nameget()
{
  return name;
}

Shape& operator=(Shape& a)
{
  strcpy(name,a.nameget());
}
//shape.h
#ifndef _shape_h
#define _shape_h
#include "showable.h"
#include "point.h"
class Shape : public Showable
{
  char name[30+1];
  public:
  Shape();
  Shape(char*);
  ostream& display(ostream&) const;
  char* nameget();
  virtual Point& centroid() const=0;
};
#endif
//showable.cpp
#include<ostream>
#include "showable.h"
using namespace std;

ostream& operator<<(ostream& os,Showable& a)
{
  a.display(os);
}
//showable.h
#ifndef _showable_h
#define _showable_h
class Showable
{
  public:
  virtual ostream& display(ostream&) const=0;

};

ostream& operator<<(ostream& os, Showable&);
#endif
//main.cpp
#include <iostream>
#include "Polygon.h"
using namespace std;
 int main() {
    Polygon rect("Rectangle", 4);
    rect(0) = Point(1, 1);
    rect(1) = Point(2, 1);
    rect(2) = Point(2, 4);
    rect(3) = Point(1, 4);

    cout << rect << endl;

    return 0;
 }
//point.cpp
#include<ostream>
#include"point.h"
using namespace std;

Point::Point()
{
  set_x(0);
  set_y(0);
}

Point::Point(double varX, double varY)
{
  set_x(varX);
  set_y(varY);
}

void Point::set_x(double var)
{
  x=var;
}

void Point::set_y(double var)
{
  y=var;
}

ostream& Point::display(ostream& os) const
{
 os << '(' << x << ',' << y << ')' <<endl;
 return os;
}

friend ostream& operator<<(ostream& os,Point& a)
{
  a.display(os);
}

double Point::xget()
{
 return x;
}
double Point::yget()
{
  return y;
}

Point& operator+(Point& b)
{
  double tempX=x()+b.x();
  double tempY=y()+b.y();
  Point c(tempX,tempY);
  return *c;
}

Point& operator/(Point& o,int value)
{
  double tempX=o.x()/value;
  double tempY=o.y()/value;
  Point c(tempX,tempY);
  return *c;
}
//point.h
#ifndef _point_h
#define _point_h
class Point : public Showable
{
double x;
double y;
public:
Point();
Point(double,double);
ostream& display(ostream&) const;
double xget();
double yget();
void set_x(double);
void set_y(double);
friend ostream& operator<<(ostream&,Point&);
};

Point& operator+(Point&);
Point& operator/(Point&,int);
#endif
//polygon.cpp
//abstract Base Classes
#include <iostream>
#include "Polygon.h"
#include "point.h"
#include "shape.h"
using namespace std;

Polygon::Polygon(char* str , int V):Shape(str)
{
  points=V;
  Point* vertex= new Point[V];
}

Point& Polygon::operator()(int i)
{
  return vertex[i];
}

int Polygon::pointsNum()
{
 return points;
}

Polygon& Polygon::operator=(Polygon& b)
{
  points=b.pointsNum();
  if(vertex)
  delete[] vertex;

  if(this!=&b && b!=NULL)
  {
    vertex=new Point[b.pointNum()];
    for(int i=0;i<b.pointNum();i++)
    {
      vertex[i]=b.vertex[i];
    }
  }
  else
  {
    vertex=NULL;
  }
  return *this;
}
ostream& Polygon::display(ostream& os) const
{
  os << (Shape)(*this) <<" Centroid " << centroid();
}

friend ostream& operator<<(ostream& os,Polygon& a)
{
  a.display(os);
  return os;
}


Point& Polygon::centroid() const
{
  int tempX=0,tempY=0;
  for(int i=0;i<points;i++)
  {
    tempX+= vertex[i].xget();
  }
  for(int i=0;i<points;i++)
  {
    tempY+= vertex[i].yget();
  }
  Point c(tempX/points,tempY/points);
  return *c;
}
//polygon.h
#ifndef _polygon_h
#define _polygon_h
#include "shape.h"
#include "point.h"

class Polygon : public Shape
{
  int points;
  Point* vertex;
  public:
  Polygon(char* , int);
  Point& operator()(int);
  Polygon& Polygon::operator=(Polygon& b);
  int pointsNum();
  ostream& display(ostream& os) const;
  Point& centroid() const;
  friend ostream& operator<<(ostream&,Polygon&);
};


#endif
//shape.cpp
#include<cstring>
#include<ostream>
#include "shape.h"
using namespace std;
Shape::Shape()
{
  name[0]='\0';
}
Shape::Shape(char* str)
{
  strcpy(name,str);
}

ostream& Shape::display(ostream& os) const
{
  os << name;
  return os;
}

char* Shape::nameget()
{
  return name;
}

Shape& operator=(Shape& a)
{
  strcpy(name,a.nameget());
}
//shape.h
#ifndef _shape_h
#define _shape_h
#include "showable.h"
#include "point.h"
class Shape : public Showable
{
  char name[30+1];
  public:
  Shape();
  Shape(char*);
  ostream& display(ostream&) const;
  char* nameget();
  virtual Point& centroid() const=0;
};
#endif
//showable.cpp
#include<ostream>
#include "showable.h"
using namespace std;

ostream& operator<<(ostream& os,Showable& a)
{
  a.display(os);
}
//showable.h
#ifndef _showable_h
#define _showable_h
class Showable
{
  public:
  virtual ostream& display(ostream&) const=0;

};

ostream& operator<<(ostream& os, Showable&);
#endif
//main.cpp

32 main.cpp point.cpp Polygon.cpp shape.cpp showable.cpp
Borland C++ 5.5.1 for Win32 Copyright (c) 1993, 2000 Borland
main.cpp:
Error E2462 showable.h 8: 'virtual' can only be used with non-template memb
er functions
Error E2303 showable.h 8: Type name expected
Error E2139 showable.h 8: Declaration missing ;
Error E2141 showable.h 12: Declaration syntax error
Error E2303 point.h 12: Type name expected
Error E2139 point.h 12: Declaration missing ;
Error E2061 point.h 17: Friends must be functions or classes
Error E2139 point.h 17: Declaration missing ;
Error E2321 point.h 18: Declaration does not specify a tag or an identifier

Error E2303 shape.h 13: Type name expected
Error E2139 shape.h 13: Declaration missing ;
Error E2303 Polygon.h 17: Type name expected
Error E2139 Polygon.h 17: Declaration missing ;
Error E2061 Polygon.h 19: Friends must be functions or classes
Error E2139 Polygon.h 19: Declaration missing ;
Error E2321 Polygon.h 20: Declaration does not specify a tag or an identifi
er
Error E2094 main.cpp 13: 'operator<<' not implemented in type 'std::ostream
' for arguments of type 'Polygon' in function main()
*** 17 errors in Compile ***
point.cpp:
Error E2303 point.h 6: Type name expected
Error E2303 point.h 12: Type name expected
Error E2139 point.h 12: Declaration missing ;
Error E2061 point.h 17: Friends must be functions or classes
Error E2139 point.h 17: Declaration missing ;
Error E2321 point.h 18: Declaration does not specify a tag or an identifier

Error E2316 point.cpp 29: 'display' is not a member of 'Point'
*** 7 errors in Compile ***
Polygon.cpp:
Error E2462 showable.h 8: 'virtual' can only be used with non-template memb
er functions
Error E2303 showable.h 8: Type name expected
Error E2139 showable.h 8: Declaration missing ;
Error E2141 showable.h 12: Declaration syntax error
Error E2303 point.h 12: Type name expected
Error E2139 point.h 12: Declaration missing ;
Error E2061 point.h 17: Friends must be functions or classes
Error E2139 point.h 17: Declaration missing ;
Error E2321 point.h 18: Declaration does not specify a tag or an identifier

Error E2303 shape.h 13: Type name expected
Error E2139 shape.h 13: Declaration missing ;
Error E2303 Polygon.h 17: Type name expected
Error E2139 Polygon.h 17: Declaration missing ;
Error E2061 Polygon.h 19: Friends must be functions or classes
Error E2139 Polygon.h 19: Declaration missing ;
Error E2321 Polygon.h 20: Declaration does not specify a tag or an identifi
er
Warning W8004 Polygon.cpp 14: 'vertex' is assigned a value that is never us
ed in function Polygon::Polygon(char *,int)
Error E2094 Polygon.cpp 32: 'operator!=' not implemented in type 'Polygon'
for arguments of type 'int' in function Polygon::operator =(Polygon &)
Error E2316 Polygon.cpp 34: 'pointNum' is not a member of 'Polygon' in func
tion Polygon::operator =(Polygon &)
Error E2316 Polygon.cpp 35: 'pointNum' is not a member of 'Polygon' in func
tion Polygon::operator =(Polygon &)
Error E2015 Polygon.cpp 46: Ambiguity between 'ostream' and 'std::ostream'
Error E2238 Polygon.cpp 46: Multiple declaration for 'ostream'
Error E2344 showable.h 12: Earlier declaration of 'ostream'
Error E2141 Polygon.cpp 46: Declaration syntax error
*** 23 errors in Compile ***
shape.cpp:
Error E2462 showable.h 8: 'virtual' can only be used with non-template memb
er functions
Error E2303 showable.h 8: Type name expected
Error E2139 showable.h 8: Declaration missing ;
Error E2141 showable.h 12: Declaration syntax error
Error E2303 point.h 12: Type name expected
Error E2139 point.h 12: Declaration missing ;
Error E2061 point.h 17: Friends must be functions or classes
Error E2139 point.h 17: Declaration missing ;
Error E2321 point.h 18: Declaration does not specify a tag or an identifier

Error E2303 shape.h 13: Type name expected
Error E2139 shape.h 13: Declaration missing ;
Error E2015 shape.cpp 16: Ambiguity between 'ostream' and 'std::ostream'
Error E2238 shape.cpp 16: Multiple declaration for 'ostream'
Error E2344 showable.h 12: Earlier declaration of 'ostream'
Error E2141 shape.cpp 16: Declaration syntax error
*** 15 errors in Compile ***
showable.cpp:
Error E2462 showable.h 8: 'virtual' can only be used with non-template memb
er functions
Error E2303 showable.h 8: Type name expected
Error E2139 showable.h 8: Declaration missing ;
Error E2141 showable.h 12: Declaration syntax error
Error E2015 showable.cpp 7: Ambiguity between 'ostream' and 'std::ostream'
Error E2238 showable.cpp 7: Multiple declaration for 'ostream'
Error E2344 showable.h 12: Earlier declaration of 'ostream'
Error E2141 showable.cpp 7: Declaration syntax error
*** 8 errors in Compile ***

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1 Answer

Editorial Team · Answer 1 · 2026-06-12T11:47:03+00:00

This member function:

class Shape : public Showable
{
public:
    virtual Point& centroid()=0;
};

is not implemented by Polygon. Polygon has the following member function declaration:

class Polygon : public Shape
{
public:
    Point& centroid() const;
};

which is a different signature to Shape::centroid() (note const) and introduces a new member function for Polygon. This means that Polygon is itself abstract as it does not provide an implementation for the pure virtual function declared in Shape.

To fix, remove the const qualifier from the declaration and the definition of Polygon::centroid() (and ensure the return type is Point& in the definition as it is not currently).

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