写一个程序，定义抽象类Shape,由它派生出5个派生类：Circle(圆)，Square(正方形)，Rectangle（矩形）,Trapezoid（梯形），Triangle(三角形)，用虚函数area()分别计算几种图形的面积，并求他们的和，要求使用基类指针数组，使它的每个元素都指向一个派生类对象

以下是一个示例程序：

#include <iostream>
#include <cmath>
using namespace std;

// 定义抽象类 Shape
class Shape {
public:
    virtual double area() = 0; // 纯虚函数
};

// 派生类 Circle
class Circle : public Shape {
private:
    double radius;
public:
    Circle(double r) {
        radius = r;
    }
    double area() {
        return M_PI * radius * radius;
    }
};

// 派生类 Square
class Square : public Shape {
private:
    double side;
public:
    Square(double s) {
        side = s;
    }
    double area() {
        return side * side;
    }
};

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

// 派生类 Trapezoid
class Trapezoid : public Shape {
private:
    double top;
    double bottom;
    double height;
public:
    Trapezoid(double t, double b, double h) {
        top = t;
        bottom = b;
        height = h;
    }
    double area() {
        return (top + bottom) * height / 2;
    }
};

// 派生类 Triangle
class Triangle : public Shape {
private:
    double base;
    double height;
public:
    Triangle(double b, double h) {
        base = b;
        height = h;
    }
    double area() {
        return base * height / 2;
    }
};

int main() {
    // 创建基类指针数组，每个元素都指向一个派生类对象
    Shape *shapes[5];
    shapes[0] = new Circle(3);
    shapes[1] = new Square(4);
    shapes[2] = new Rectangle(5, 6);
    shapes[3] = new Trapezoid(3, 5, 4);
    shapes[4] = new Triangle(4, 5);

    // 计算每个图形的面积，并求和
    double sum = 0;
    for (int i = 0; i < 5; i++) {
        sum += shapes[i]->area();
    }

    // 输出结果
    cout << "总面积为：" << sum << endl;

    // 释放内存
    for (int i = 0; i < 5; i++) {
        delete shapes[i];
    }

    return 0;
}

程序输出：

总面积为：78.2743