定义一个基类Shape，在此基础上派生出Rectangle和Circle，二者都有getArea( )函数计算对象的面积，再使用Rectangle类创建一个派生类Square。在主函数中测试Rectangle、Circle和Square这三个类。

以下是实现代码：

#include<iostream>
using namespace std;

class Shape {
public:
    virtual double getArea() = 0; //纯虚函数，必须在派生类中实现
};

class Rectangle : public Shape {
private:
    double length;
    double width;
public:
    Rectangle(double l, double w) {
        length = l;
        width = w;
    }
    double getArea() {
        return length * width;
    }
};

class Circle : public Shape {
private:
    double radius;
public:
    Circle(double r) {
        radius = r;
    }
    double getArea() {
        return 3.14 * radius * radius;
    }
};

class Square : public Rectangle {
public:
    Square(double side) : Rectangle(side, side) {}
};

int main() {
    Rectangle rect(3, 4);
    Circle circle(5);
    Square square(6);

    cout << "The area of the rectangle is " << rect.getArea() << endl;
    cout << "The area of the circle is " << circle.getArea() << endl;
    cout << "The area of the square is " << square.getArea() << endl;

    return 0;
}

输出结果：

The area of the rectangle is 12
The area of the circle is 78.5
The area of the square is 36