C++状态模式：State Pattern

状态模式：允许对象在其内部状态改变时改变它的行为，对象看起来好像修改类它的类。

状态模式是有限状态机（Finite State Machine）的一种实现方式。

我们都知道，http请求报文由请求行、请求头、请求空行和请求体四部分组成，服务器在解析请求报文时，会根据当前所在行的状态来进行不同方式的解析（逐行解析）。

类图：

实现如下：

state.h：

#ifndef STATE_H
#define STATE_H
#include <iostream>
using namespace std;

class State {
public:
    virtual void work() = 0;
};
#endif // STATE_H

requestLine.h：

#ifndef REQUESTLINE_H
#define REQUESTLINE_H
#include "state.h"

class RequestLine : public State {
public:
    virtual void work() override
    {
        cout << "解析请求行" << endl;
    }
};

#endif // REQUESTLINE_H

requestHeader.h：

#ifndef REQUESTHEADER_H
#define REQUESTHEADER_H
#include "state.h"

class RequestHeader : public State {
public:
    virtual void work() override
    {
        cout << "解析请求头" << endl;
    }
};
#endif // REQUESTHEADER_H

requestBlankLine.h：

#ifndef REQUESTBLANKLINE_H
#define REQUESTBLANKLINE_H
#include "state.h"

class RequestBlankLine : public State {
public:
    virtual void work() override
    {
        cout << "解析请求空行" << endl;
    }
};
#endif // REQUESTBLANKLINE_H

requestBody.h：

#ifndef REQUESTBODY_H
#define REQUESTBODY_H
#include "state.h"

class RequestBody : public State {
public:
    virtual void work() override
    {
        cout << "解析请求体" << endl;
    }
};
#endif // REQUESTBODY_H

httpAnalyze.h：

#ifndef HTTPANALYZE_H
#define HTTPANALYZE_H
#include "requestLine.h"
#include "requestHeader.h"
#include "requestBlankLine.h"
#include "requestBody.h"

class HttpAnalyze {
public:
    void setCurState(State* s)
    {
        m_curState = s;
    }

    void analyze()
    {
        m_curState->work();
    }
private:
    State* m_line = new RequestLine();
    State* m_header = new RequestHeader();
    State* m_blankLine = new RequestBlankLine();
    State* m_body = new RequestBody();
    State* m_curState;
};

#endif // HTTPANALYZE_H

main.cpp：

/*
 * 状态模式
 *
 * date:2023-9-16
*/

#include "httpAnalyze.h"

int main()
{
    State* line = new RequestLine();
    State* header = new RequestHeader();
    State* blankLine = new RequestBlankLine();
    State* body = new RequestBody();

    HttpAnalyze* h = new HttpAnalyze();

    cout << "开始解析请求报文" << endl;

    h->setCurState(line);
    h->analyze();

    h->setCurState(header);
    h->analyze();

    h->setCurState(blankLine);
    h->analyze();

    h->setCurState(body);
    h->analyze();

    cout << "解析完毕" << endl;

}

运行结果：