如果对象不同状态会去执行不同的命令，并跳转到另一个状态，那么这个状态可以抽象出一个基类

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#include <iostream>
#include <memory>
#include <string>

class MusicPlayer;
class PlayerStatue {
public:
    virtual ~PlayerStatue() {}
    virtual void play(MusicPlayer &player) = 0;
    virtual void pause(MusicPlayer &player) = 0;
    virtual void stop(MusicPlayer &player) = 0;
};

class MusicPlayer {
public:
    void setState(std::unique_ptr<PlayerStatue> state) {
        m_state = std::move(state);
    }

    void play() {
        m_state->play(*this);
    }
    void pause() {
        m_state->pause(*this);
    }
    void stop() {
        m_state->stop(*this);
    }
private:
    std::unique_ptr<PlayerStatue> m_state;
};


class PlayingState : public PlayerStatue {
public:
    void play(MusicPlayer &player) override;
    void pause(MusicPlayer &player) override;
    void stop(MusicPlayer &player) override;
};
class PauseState : public PlayerStatue {
public:
    void play(MusicPlayer &player) override;
    void pause(MusicPlayer &player) override;
    void stop(MusicPlayer &player) override;
};
class StopState : public PlayerStatue {
public:
    void play(MusicPlayer &player) override;
    void pause(MusicPlayer &player) override;
    void stop(MusicPlayer &player) override;
};


void PlayingState::play(MusicPlayer &player) {
    std::cout << "已经在播放状态了" << std::endl;
}
void PlayingState::pause(MusicPlayer &player) {
    std::cout << "暂停" << std::endl;
    player.setState(std::make_unique<PauseState>());
}
void PlayingState::stop(MusicPlayer &player) {
    std::cout << "停止" << std::endl;
    player.setState(std::make_unique<StopState>());
}

void PauseState::play(MusicPlayer &player) {
    std::cout << "播放" << std::endl;
    player.setState(std::make_unique<PlayingState>());
}
void PauseState::pause(MusicPlayer &player) {
    std::cout << "已经在暂停状态了" << std::endl;
}
void PauseState::stop(MusicPlayer &player) {
    std::cout << "停止" << std::endl;
    player.setState(std::make_unique<StopState>());
}

void StopState::play(MusicPlayer &player) {
    std::cout << "播放" << std::endl;
    player.setState(std::make_unique<PlayingState>());
}
void StopState::pause(MusicPlayer &player) {
    std::cout << "无正在播放" << std::endl;
}
void StopState::stop(MusicPlayer &player) {
    std::cout << "已经在停止状态了" << std::endl;
}


int main() {
    MusicPlayer player;

    player.setState(std::make_unique<PlayingState>());

    player.pause();
    player.pause();
    player.play();
    player.stop();

    return 0;
}