C ＋＋编写一个程序，包含抽象基类 Shape 和其派生类 Circle、 Square、 Rectangle 和 Triangle。用虛函数实现这几种图形的面积的计算，要求在主两数中用抽象类的指针调用函 数来输出相应图形的面积。

以下是一个示例程序，其中抽象基类 Shape 定义了纯虚函数 area()，表示计算图形面积的方法，而 Circle、Square、Rectangle 和 Triangle 派生自 Shape 类，并实现了各自的 area() 方法。

#include <iostream>
using namespace std;

class Shape {
public:
    virtual double area() = 0; // 纯虚函数，用于计算图形面积
};

class Circle : public Shape {
private:
    double radius;

public:
    Circle(double r) {
        radius = r;
    }

    double area() {
        return 3.14 * radius * radius;
    }
};

class Square : public Shape {
private:
    double side;

public:
    Square(double s) {
        side = s;
    }

    double area() {
        return side * side;
    }
};

class Rectangle : public Shape {
private:
    double width;
    double height;

public:
    Rectangle(double w, double h) {
        width = w;
        height = h;
    }

    double area() {
        return width * height;
    }
};

class Triangle : public Shape {
private:
    double base;
    double height;

public:
    Triangle(double b, double h) {
        base = b;
        height = h;
    }

    double area() {
        return 0.5 * base * height;
    }
};

int main() {
    Shape *shape;

    shape = new Circle(5);
    cout << "圆的面积：" << shape->area() << endl;

    shape = new Square(4);
    cout << "正方形的面积：" << shape->area() << endl;

    shape = new Rectangle(3, 5);
    cout << "矩形的面积：" << shape->area() << endl;

    shape = new Triangle(4, 5);
    cout << "三角形的面积：" << shape->area() << endl;

    return 0;
}

运行结果：

圆的面积：78.5
正方形的面积：16
矩形的面积：15
三角形的面积：10