编程计算图形的面积。程序可以用来计算圆、长方形、正方形的面积，运行时先提示用户选择图形的类型，然后对圆形要求输入半径值，对长方形要求输入长和宽的值，对正方形要求输入边长的值 计算面积并将其输出。给出C++代码

在C++中，你可以创建一个函数库来处理图形的面积计算。这里是一个简单的示例，展示了如何实现这个功能：

#include <iostream>
#include <cmath>

// 定义一个抽象基类Shape
class Shape {
public:
    virtual double getArea() const = 0; // 纯虚函数，用于声明所有形状都有一个getArea方法
};

// 具体形状类
class Circle : public Shape {
private:
    double radius;
public:
    Circle(double r) : radius(r) {}
    double getArea() const override { return M_PI * pow(radius, 2); }
};

class Rectangle : public Shape {
private:
    double length, width;
public:
    Rectangle(double l, double w) : length(l), width(w) {}
    double getArea() const override { return length * width; }
};

class Square : public Shape {
private:
    double side;
public:
    Square(double s) : side(s) {}
    double getArea() const override { return pow(side, 2); }
};

int main() {
    std::cout << "Please choose a shape:\n";
    std::cout << "1. Circle\n2. Rectangle\n3. Square\n";
    int choice;
    std::cin >> choice;

    Shape* shape;
    switch (choice) {
        case 1:
            double radius;
            std::cout << "Enter the radius of the circle: ";
            std::cin >> radius;
            shape = new Circle(radius);
            break;
        case 2:
            double length, width;
            std::cout << "Enter the length and width of the rectangle: ";
            std::cin >> length >> width;
            shape = new Rectangle(length, width);
            break;
        case 3:
            double side;
            std::cout << "Enter the side length of the square: ";
            std::cin >> side;
            shape = new Square(side);
            break;
        default:
            std::cerr << "Invalid choice!\n";
            return 1;
    }

    double area = shape->getArea();
    std::cout << "The area is: " << area << "\n";

    delete shape; // 释放内存
    return 0;
}