抽象基类（abstract base class,ABC）

例子：圆与椭圆。建立一个基类BaseEllipse，建立它的恋歌继承了Ellipse和Circle。

ellipse.h

#ifndef ELLIPSE_H_#define ELLIPSE_H_class BaseEllipse{public:    virtual double Area() const =0;};class Ellipse : public BaseEllipse{private:    double a, b;public:    Ellipse(double a, double b) : a(a), b(b) {}    virtual double Area() const;};class Circle : public BaseEllipse{private:    double r;public:    Circle(double r) : r(r) { }    virtual double Area() const;};#endif // ELLIPSE_H_

ellipse.cpp

#include "ellipse.h"double Ellipse::Area() const{    return 3.14159 * a * b;}double Circle::Area() const{    return 3.14159 * r * r;}

测试类

#include 
     
      #include "ellipse.h"int main(){    using std::cout;    using std::endl;    double a = 1.0, b = 2.0, r = 3.0;    Ellipse * ellipse = new Ellipse(a, b);    cout << ellipse->Area() << endl;    Circle * circle = new Circle(r);    cout << circle->Area() << endl;    return 0;}
     

输出效果：

6.2831828.2743