设计一个图形类（Shape），由它派生出5个派生类:三角形类（Triangle）、正方形类（Square）、圆形类（Circle）、矩形类（Rectangle）、梯形类（triangle）类，利用虚函数计算图形面积，用一个函数printArea分别输出以上5者的面积。

好的，以下是实现该题目的 C++ 代码：

#include<iostream>
using namespace std;

class Shape {  // 基类 Shape
public:
    virtual double getArea() { return 0; }  // 定义虚函数计算面积
};

class Triangle : public Shape {  // 三角形类
private:
    double base;
    double height;
public:
    Triangle(double b, double h) : base(b), height(h) {}
    virtual double getArea() { return 0.5 * base * height; }
};

class Square : public Shape {  // 正方形类
private:
    double side;
public:
    Square(double s) : side(s) {}
    virtual double getArea() { return side * side; }
};

class Circle : public Shape {  // 圆形类
private:
    double radius;
public:
    Circle(double r) : radius(r) {}
    virtual double getArea() { return 3.14 * radius * radius; }
};

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

class Trapezoid : public Shape {  // 梯形类
private:
    double upperBase;
    double lowerBase;
    double height;
public:
    Trapezoid(double ub, double lb, double h) : upperBase(ub), lowerBase(lb), height(h) {}
    virtual double getArea() { return (upperBase + lowerBase) * height / 2; }
};

void printArea(Shape* shape) {  // 输出各个图形的面积
    if (Triangle* t = dynamic_cast<Triangle*>(shape)) {
        cout << "Triangle area: " << t->getArea() << endl;
    } else if (Square* s = dynamic_cast<Square*>(shape)) {
        cout << "Square area: " << s->getArea() << endl;
    } else if (Circle* c = dynamic_cast<Circle*>(shape)) {
        cout << "Circle area: " << c->getArea() << endl;
    } else if (Rectangle* r = dynamic_cast<Rectangle*>(shape)) {
        cout << "Rectangle area: " << r->getArea() << endl;
    } else if (Trapezoid* tp = dynamic_cast<Trapezoid*>(shape)) {
        cout << "Trapezoid area: " << tp->getArea() << endl;
    } else {
        cout << "Unknown shape!" << endl;
    }
}

int main() {
    Shape* shapes[5];
    shapes[0] = new Triangle(5, 6);
    shapes[1] = new Square(4);
    shapes[2] = new Circle(3);
    shapes[3] = new Rectangle(7, 8);
    shapes[4] = new Trapezoid(4, 6, 5);

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

    return 0;
}

程序的输出结果为：

Triangle area: 15
Square area: 16
Circle area: 28.26
Rectangle area: 56
Trapezoid area: 25