c++定义一个抽象基类Shape，由它派生出3个派生类：Circle，Square，Rectangle，用一个函数printarea分别输出以上三者的面积，3个图形的数据在定义对象时给定。创建基类指针，指向各个派生类对象

好的，下面是 C++ 的代码示例：

#include <iostream>

using namespace std;

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

class Circle : public Shape {
public:
    Circle(double r) : radius(r) {}
    double area() const override {
        return 3.14 * 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 l, double w) : length(l), width(w) {}
    double area() const override {
        return length * width;
    }
private:
    double length;
    double width;
};

void printarea(const Shape& shape) {
    cout << "The area is " << shape.area() << endl;
}

int main() {
    Circle circle(5);
    Square square(4);
    Rectangle rectangle(3, 6);

    printarea(circle);
    printarea(square);
    printarea(rectangle);

    Shape* shape_ptrs[3];
    shape_ptrs[0] = new Circle(2);
    shape_ptrs[1] = new Square(3);
    shape_ptrs[2] = new Rectangle(4, 5);

    for (int i = 0; i < 3; i++) {
        printarea(*shape_ptrs[i]);
        delete shape_ptrs[i];
    }

    return 0;
}

运行结果如下：

The area is 78.5
The area is 16
The area is 18
The area is 12.56
The area is 9
The area is 20