编写程序定义抽象类Shape，由它派生的五个类Circle Square Rectangle Trapezoid Triangle 用虚函数分别计算几种图形的面积，并求它们的和。要求用基类指针数组，使它每一个元素指向一个派生类对象

以下是实现代码：

#include <iostream>
using namespace std;

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

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

class Square : public Shape {
public:
    Square(double l) : length(l) {}
    virtual double area() { return length * length; }
private:
    double length;
};

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

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

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

int main() {
    Shape* shapes[5];
    shapes[0] = new Circle(2.0);
    shapes[1] = new Square(3.0);
    shapes[2] = new Rectangle(2.0, 3.0);
    shapes[3] = new Trapezoid(2.0, 4.0, 3.0);
    shapes[4] = new Triangle(3.0, 4.0);

    double totalArea = 0.0;
    for(int i=0; i<5; i++) {
        totalArea += shapes[i]->area();
    }

    cout << "Total area: " << totalArea << endl;

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

    return 0;
}

程序中定义了一个抽象类Shape，它包含一个纯虚函数area()，表示求面积。然后定义了五个派生类：Circle、Square、Rectangle、Trapezoid和Triangle，它们分别实现了Shape类的area()函数。在main函数中，定义了一个Shape类型的指针数组shapes，用于存储各个派生类的对象。通过循环遍历shapes数组，可以依次计算出每个图形的面积，并将它们加起来得到总面积。最后，需要释放掉每个对象的内存空间。