3. (程序题, 35分) 请编程实现：定义一个基类Shape，在此基础上派生出Rectangle和Circle，三者都有getArea()函数计算对象的面积。使用Rectangle类创建一个派生类Square。注意基本数据类型使用float类型。 其中主函数如下： int main() { Shape s1; cout << "The area of the Shape is " << s1.getArea () << endl; Rectangle s2(4,6); cout << "The area of the Rectangle is " << s2.getArea() << endl; Square s3(5); cout << "The area of the Square is " << s3.getArea() << endl; return 0; } 程序的运行结果如下：

以下是题目的代码实现：

#include<iostream>
using namespace std;

class Shape{
public:
    virtual float getArea(){
        return 0;
    }
};

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

class Circle: public Shape{
private:
    float radius;
public:
    Circle(float r){
        radius = r;
    }
    virtual float getArea(){
        return 3.1415 * radius * radius;
    }
};

class Square: public Rectangle{
public:
    Square(float a): Rectangle(a,a){}
};

int main(){
    Shape s1;
    cout << "The area of the Shape is " << s1.getArea() << endl;

    Rectangle s2(4,6);
    cout << "The area of the Rectangle is " << s2.getArea() << endl;

    Square s3(5);
    cout << "The area of the Square is " << s3.getArea() << endl;

    return 0;
}

程序的输出结果：

The area of the Shape is 0
The area of the Rectangle is 24
The area of the Square is 25

其中，基类Shape中的getArea()函数返回0，因为无法计算Shape的面积。派生类Rectangle和Circle分别计算矩形和圆的面积，Square继承自Rectangle，并重写了构造函数，使得正方形的长和宽相等，从而实现了正方形的面积计算。