20.责任链模式（Chain of Responsibility Pattern）-代码实现

我们通过设计一个 审批系统 来演示责任链模式。在这个系统中，多个审批者（如经理、总监、CEO）按顺序处理请假申请，每个审批者可以根据自己的权限决定是否批准请求。如果不能处理，请求将传递给下一个审批者。

1. 定义处理者接口（Handler）

#include <iostream>
#include <string>
using namespace std;

// 处理者接口
class Approver {
protected:
    Approver* nextApprover;  // 下一个处理者

public:
    Approver() : nextApprover(nullptr) {}

    void setNext(Approver* next) {
        nextApprover = next;  // 设置下一个处理者
    }

    virtual void handleRequest(int days) = 0;  // 处理请求

    virtual ~Approver() {}
};

• Approver 是处理者接口，定义了 setNext() 方法来设置下一个处理者，并定义了 handleRequest() 方法来处理请求。

2. 定义具体处理者类（ConcreteHandler）

// 经理审批者
class Manager : public Approver {
public:
    void handleRequest(int days) override {
        if (days <= 2) {
            cout << "Manager approved the leave request for " << days << " days." << endl;
        } else if (nextApprover) {
            nextApprover->handleRequest(days);  // 转发请求给下一个处理者
        } else {
            cout << "Manager cannot approve the leave request for " << days << " days." << endl;
        }
    }
};

// 总监审批者
class Director : public Approver {
public:
    void handleRequest(int days) override {
        if (days <= 5) {
            cout << "Director approved the leave request for " << days << " days." << endl;
        } else if (nextApprover) {
            nextApprover->handleRequest(days);  // 转发请求给下一个处理者
        } else {
            cout << "Director cannot approve the leave request for " << days << " days." << endl;
        }
    }
};

// CEO审批者
class CEO : public Approver {
public:
    void handleRequest(int days) override {
        cout << "CEO approved the leave request for " << days << " days." << endl;
    }
};

• Manager、Director 和 CEO 都是具体的处理者类，继承自 Approver 并实现 handleRequest() 方法。
• 每个处理者根据请求的天数来决定是否处理请求。如果一个处理者无法处理请求，它将请求传递给下一个处理者。

3. 客户端调用

int main() {
    // 创建处理者
    Approver* manager = new Manager();
    Approver* director = new Director();
    Approver* ceo = new CEO();

    // 设置责任链
    manager->setNext(director);
    director->setNext(ceo);

    // 客户端发送请求
    cout << "Requesting leave for 1 day:" << endl;
    manager->handleRequest(1);  // Manager handles

    cout << "\nRequesting leave for 3 days:" << endl;
    manager->handleRequest(3);  // Director handles

    cout << "\nRequesting leave for 7 days:" << endl;
    manager->handleRequest(7);  // CEO handles

    // 清理内存
    delete manager;
    delete director;
    delete ceo;

    return 0;
}

4. 输出结果

Requesting leave for 1 day:
Manager approved the leave request for 1 days.

Requesting leave for 3 days:
Director approved the leave request for 3 days.

Requesting leave for 7 days:
CEO approved the leave request for 7 days.

• 在这个例子中，客户端请求不同天数的请假。请求依次传递给 Manager、Director、CEO 进行处理，直到找到合适的处理者。