C++/Class/Abstract Class

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abstract base class

  
#include <iostream>
using namespace std;
class CPolygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }
    virtual int area (void) =0;
};
class CRectangle: public CPolygon {
  public:
    int area (void)
      { return (width * height); }
};
class CTriangle: public CPolygon {
  public:
    int area (void)
      { return (width * height / 2); }
};
int main () {
  CRectangle rect;
  CTriangle trgl;
  CPolygon * ppoly1 = &rect;
  CPolygon * ppoly2 = &trgl;
  ppoly1->set_values (4,5);
  ppoly2->set_values (4,5);
  cout << ppoly1->area() << endl;
  cout << ppoly2->area() << endl;
  return 0;
}


Create an abstract class.

 

#include <iostream>
using namespace std;
class area {
  double dim1, dim2; 
public:
  void setarea(double d1, double d2)
  {
    dim1 = d1;
    dim2 = d2;
  }
  void getdim(double &d1, double &d2)
  {
    d1 = dim1;
    d2 = dim2;
  }
  virtual double getarea() = 0; // pure virtual function
};
class rectangle : public area {
public:
  double getarea() 
  {
    double d1, d2;
  
    getdim(d1, d2);
    return d1 * d2;
  }
};
class triangle : public area {
public:
  double getarea()
  {
    double d1, d2;
  
    getdim(d1, d2);
    return 0.5 * d1 * d2;
  }
};
int main()
{
  area *p;
  rectangle r;
  triangle t;
  r.setarea(3.3, 4.5);
  t.setarea(4.0, 5.0);
  p = &r;
  cout << "Rectangle has area: " << p->getarea() << "\n";
  p = &t;
  cout << "Triangle has area: " << p->getarea() << "\n";
  return 0;
}


pure virtual members can be called from the abstract base class

  
#include <iostream>
using namespace std;
class CPolygon {
  protected:
    int width, height;
  public:
    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); }
};
class CTriangle: public CPolygon {
  public:
    int area (void)
    { return (width * height / 2); }
};
int main () {
  CRectangle rect;
  CTriangle trgl;
  CPolygon * ppoly1 = &rect;
  CPolygon * ppoly2 = &trgl;
  ppoly1->set_values (4,5);
  ppoly2->set_values (4,5);
  ppoly1->printarea();
  ppoly2->printarea();
  return 0;
}
Источник — «http://cppe.ru/index.php?title=C%2B%2B/Class/Abstract_Class&oldid=1305»