意图

当一个对象的内部状态改变时，改变其行为。
适用于对象的行为由它的状态决定时，或是一个方法中有多个分支，这些分支都依赖于对象的状态。

示例

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class ServerState {
public:
    virtual void connect() = 0;
};

class Established : public ServerState {
public:
    ~Established() = default;
    
    static Established& instance() {
        static Established instance;
        return instance;
    }

    void connect() override {
        cout << "connection is established.\n";
    }

private:
    Established() = default;
};

class Listening : public ServerState {
public:
    ~Listening() = default;
    
    static Listening& instance() {
        static Listening instance;
        return instance;
    }

    void connect() override {
        cout << "connection is listening.\n";
    }

private:
    Listening() = default;
};

class Closed : public ServerState {
public:
    ~Closed() = default;
    
    static Closed& instance() {
        static Closed instance;
        return instance;
    }

    void connect() override {
        cout << "connection is closed.\n";
    }

private:
    Closed() = default;
};

class Server {
public:

public:
    Server() : m_state(&Closed::instance()) { }

    void changeState(ServerState* s) {
        m_state = s;
    }

    void connect() {
        m_state->connect();
    }

private:
    ServerState* m_state;
};


int main() {
    Server s;
    s.connect();

    s.changeState(&Listening::instance());
    s.connect();

    s.changeState(&Closed::instance());
    s.connect();

    return 0;
}

例如，一个服务器可能有正在监听、已建立连接、服务关闭等状态，当调用连接时，每种状态都会有不同的表示，我们分别使用单例类来表现。当切换状态后，服务被调用同样的连接方法，可以得到不同的结果：

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connection is closed.
connection is listening.
connection is closed.