用C++编写一个程序满足已知抽象类Shape及Shape类的3个公有派生类圆类Circle、矩形类Rectangle、三角形类Triangle声明如下： class Shape { public: virtual double area() const = 0; }; class Circle:public Shape { public: Circle(double r); virtual double area() const; protected: double m_radius; }; class Rectangle:public Shape { public: Rectangle(double w, double h); virtual double area() const; protected: double m_width, m_height; }; class Triangle:public Shape { public: Triangle(double a, double b, double c); virtual double area() const; protected: double m_a, m_b, m_c; }; 试完成成员函数的定义，在主函数中输入圆的半径、矩形的宽和高、三角形的三条边，定义对象，利用函数void printArea(const Shape &s)输出圆、矩形及三角形的面积。 输入 三行，第1行圆的半径，第2行矩形的宽和高，第3行三角形的三条边。 输出 圆、矩形及三角形的面积(取2位小数)。

以下是代码实现：

#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

class Shape {
public:
    virtual double area() const = 0;
};

class Circle:public Shape {
public:
    Circle(double r) {
        m_radius = r;
    }
    virtual double area() const {
        return 3.14 * m_radius * m_radius;
    }
protected:
    double m_radius;
};

class Rectangle:public Shape {
public:
    Rectangle(double w, double h) {
        m_width = w;
        m_height = h;
    }
    virtual double area() const {
        return m_width * m_height;
    }
protected:
    double m_width, m_height;
};

class Triangle:public Shape {
public:
    Triangle(double a, double b, double c) {
        m_a = a;
        m_b = b;
        m_c = c;
    }
    virtual double area() const {
        double p = (m_a + m_b + m_c) / 2.0;
        return sqrt(p * (p - m_a) * (p - m_b) * (p - m_c));
    }
protected:
    double m_a, m_b, m_c;
};

void printArea(const Shape &s) {
    cout << fixed << setprecision(2) << "面积为：" << s.area() << endl;
}

int main() {
    double r, w, h, a, b, c;
    cin >> r >> w >> h >> a >> b >> c;

    Circle circle(r);
    Rectangle rectangle(w, h);
    Triangle triangle(a, b, c);

    cout << "圆的";
    printArea(circle);

    cout << "矩形的";
    printArea(rectangle);

    cout << "三角形的";
    printArea(triangle);

    return 0;
}

输入示例：

2
3 4
3 4 5

输出示例：

圆的面积为：12.56
矩形的面积为：12.00
三角形的面积为：6.00