C++ 写一个状态模式的完整代码

状态模式是一种行为设计模式，它封装了对象在其生命周期的不同状态，并通过改变状态来改变对象的行为。在C++中，我们可以创建一个基类State，然后定义一组子类表示不同的状态，以及一个Context类来维护状态并触发相应的操作。

下面是一个简单的状态模式实现示例：

#include <iostream>
enum class ConnectionState { Disconnected, Connecting, Online };

class ConnectionStateMachine {
private:
    ConnectionState state;
public:
    ConnectionStateMachine() : state(ConnectionState::Disconnected) {}

    // 状态接口
    virtual void connect() = 0;
    virtual void disconnect() = 0;
    virtual void sendData() = 0;

    // 抽象状态实现
    class DisconnectedState : public ConnectionStateMachine {
    public:
        void connect() override {
            std::cout << "Connecting..." << std::endl;
            state = ConnectionState::Connecting;
        }

        void disconnect() override {
            std::cout << "Disconnected" << std::endl;
            state = ConnectionState::Disconnected;
        }

        void sendData() override {
            throw std::runtime_error("Not connected");
        }
    };

    class ConnectingState : public ConnectionStateMachine {
    public:
        void connect() override {
            std::cout << "Already connecting" << std::endl;
        }

        void disconnect() override {
            state = ConnectionState::Disconnected;
        }

        void sendData() override {
            std::cout << "Sending data in connecting state" << std::endl;
        }
    };

    class OnlineState : public ConnectionStateMachine {
    public:
        void connect() override {
            std::cout << "Already online" << std::endl;
        }

        void disconnect() override {
            state = ConnectionState::Disconnected;
        }

        void sendData() override {
            std::cout << "Sending data" << std::endl;
        }
    };

    // 根据当前状态选择正确的状态对象
    void setState(ConnectionState newState) {
        if (newState == state)
            return;
        switch (state) {
        case ConnectionState::Disconnected:
            state = ConnectionState::Connecting;
            break;
        case ConnectionState::Connecting:
            state = ConnectionState::Online;
            break;
        default:
            // 如果尝试切换到其他非预期状态，保持连接状态
            break;
        }
        state = newState;
        if (state == ConnectionState::Connecting) {
            ConnectingState *connecting = new ConnectingState();
            delete this;
            connecting->setState(state);
        } else if (state == ConnectionState::Online) {
            OnlineState *online = new OnlineState();
            delete this;
            online->setState(state);
        }
    }
};

int main() {
    ConnectionStateMachine machine;
    machine.setState(ConnectionState::Connecting);
    machine.connect();       // 输出: Already connecting
    machine.sendData();     // 输出: Sending data in connecting state

    machine.setState(ConnectionState::Online);
    machine.disconnect();   // 输出: Disconnecting...

    return 0;
}

在这个例子中，`ConnectionStateMachine`是上下文，`DisconnectedState`, `ConnectingState`, 和 `OnlineState` 分别代表不同的状态，每个状态都有对应的处理行为。在main函数中，我们通过`setState`动态地改变了状态机的状态。