编写一个程序，定义抽象基类Shape，由它派生出5个派生类：Circle、Square、Rectangle、Trapezoid、Triangle。用虛函数分别计算几种图形面积，并求它们之和

好的，这是一个经典的面向对象程序设计问题。以下是一个示例代码：

#include <iostream>
#include <vector>

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

class Circle : public Shape {
public:
    Circle(double r) : radius(r) {}
    double area() const override {
        return 3.14159 * radius * radius;
    }
private:
    double radius;
};

class Square : public Shape {
public:
    Square(double s) : side(s) {}
    double area() const override {
        return side * side;
    }
private:
    double side;
};

class Rectangle : public Shape {
public:
    Rectangle(double w, double h) : width(w), height(h) {}
    double area() const override {
        return width * height;
    }
private:
    double width, height;
};

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

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

int main() {
    std::vector<Shape*> shapes;
    shapes.push_back(new Circle(2.0));
    shapes.push_back(new Square(3.0));
    shapes.push_back(new Rectangle(2.0, 3.0));
    shapes.push_back(new Trapezoid(2.0, 4.0, 3.0));
    shapes.push_back(new Triangle(2.0, 3.0));

    double total_area = 0.0;
    for (auto shape : shapes) {
        total_area += shape->area();
    }

    std::cout << "Total area: " << total_area << std::endl;

    for (auto shape : shapes) {
        delete shape;
    }

    return 0;
}

这个程序定义了一个抽象基类 Shape，它有一个纯虚函数 area()，因此不能被实例化。然后它派生出了五个具体的形状类，并实现了它们的 area() 函数。在主函数中，我们创建了一个 Shape 指针的 vector，向其中添加了五个具体形状的实例。然后通过循环计算它们的面积，并输出总面积。最后需要释放内存，避免内存泄漏。