C ++抽象基类和继承

Question

我试图让该程序进行编译，但是我遇到了各种各样的怪异错误，即我在说抽象基类的瞬间。 但是我相信这只是因为我在声明派生类时试图创建抽象的基类的实例。

我也对某些方法获得了多个声明。

非常感谢你的帮助。 我很迷失。

我包含所有内容的原因是因为我真的不知道是什么引起了问题。 如果您想具体查看我收到错误的文件，错误消息在底部。

//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 ***

Answer 1

该成员函数：

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

不是由Polygon实现的。 Polygon具有以下成员函数声明：

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

这与Shape::centroid() （请注意const ）的签名不同，并为Polygon引入了新的成员函数。 这意味着Polygon本身是抽象的，因为它不提供Shape声明的纯virtual函数的实现。

要解决此问题，请从Polygon::centroid()的声明和定义中删除const限定符（并确保返回类型在该定义中为Point& ，因为当前不是）。

Answer 2

你有

 virtual Point& centroid()=0;

在Shape和

 virtual Point& centroid() const;

在Polygon -不同的功能。 cv限定词是功能签名的一部分。

Answer 3

class Showable
{
  public:
  virtual ostream& display(ostream&) const=0;
  /**/
  ostream& operator<<(ostream& os);
};

这里有一个潜在的问题：您确实应该为此类提供一个虚拟析构函数。 有一天，某人（可能是您）将尝试删除从基类指针从Showable派生的类的实例。 如果类具有任何虚拟方法，则最好将析构函数设为虚拟。