CRTP静态多态

一、动态多态

• 动态多态通过虚函数和继承关系来实现，执行动态绑定，在通过基类的指针或者引用调用虚函数运行时确定。

#include <iostream>
#include <cmath>

class Shape {
public:
    virtual double computeArea() const = 0;  // 纯虚函数
    virtual ~Shape() = default;  // 虚析构函数
};

class Circle : public Shape {
public:
    Circle(double radius) : radius(radius) {}

    double computeArea() const override {
        double area = M_PI * radius * radius;
        return area;
    }

private:
    double radius;
};

class Rectangle : public Shape {
public:
    Rectangle(double width, double height) : width(width), height(height) {}
    
    double computeArea() const override {
        double area = width * height;
        return area;
    }

private:
    double width, height;
};

int main() {
    Shape* circle = new Circle(5.0);
    // 输出：Circle area: 78.5398
    std::cout << "Circle area: " << circle->computeArea() << std::endl;
    Shape* rectangle = new Rectangle(4.0, 6.0);
    // 输出：Rectangle area: 24
    std::cout << "Rectangle area: " << rectangle->computeArea() << std::endl;

    delete circle;
    delete rectangle;
    return 0;
}

二、CRTP静态多态

1、CRTP多态简介

• CRTP(Curiously Recurring Template Pattern，奇异递归模板模式)是C++的一种设计模式，特点是利用模板和继承，在基类关联派生类模板参数，来实现静态多态性，不需要使用虚函数实现多态特性。

2、定义基类

• 定义一个模板基类Shape，接收一个派生类作为模板参数。

#include <iostream>
#include <cmath>

template <typename Derived>
class Shape {
public:
    double area() {
        return static_cast<Derived*>(this)->computeArea();
    }
};

• Shape基类中，area方法内部调用了派生类的computeArea方法，用static_cast可以确保在编译时进行类型检查。

3、定义派生类

• 定义两个派生类Circle和Rectangle。

class Circle : public Shape<Circle> {
public:
    Circle(double radius) : radius(radius) {}

    double computeArea() {
        double area = M_PI * radius * radius;
        return area;
    }

private:
    double radius;
};

class Rectangle : public Shape<Rectangle> {
public:
    Rectangle(double width, double height) : width(width), height(height) {}

    double computeArea() {
        double area = width * height;
        return area;
    }

private:
    double width, height;
};

• 每个派生类必须实现computeArea方法。

4、CRTP使用

int main() {
    Circle circle(5.0);
    // 输出：Circle area: 78.5398
    std::cout << "Circle area: " << circle.area() << std::endl;
    Rectangle rectangle(4.0, 6.0);
    // 输出：Rectangle area: 24
    std::cout << "Rectangle area: " << rectangle.area() << std::endl;
    return 0;
}

• 创建实例，计算面积。