C++中的抽象基类示例

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 <iostream>
#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：

6.28318
28.2743