C++ 状态模式异常

Question

我是 C++ 新手！请帮助我实现状态模式。

我有以下代码，但无法编译：

class MasterThreadState;
class Init; class Idle;

class MasterThread
{
public:
    MasterThread(){
        _state = Init::State();
    }
    void handle(int sender, int tag);
private:
    int _sender;
    MasterThreadState* _state;
private:
    friend class MasterThreadState;
    void goTo(MasterThreadState* newState){ _state = newState; }
};

class MasterThreadState
{
public:
    virtual void recieved(MasterThread& master, int tag);
protected:
    void goTo(MasterThread& m, MasterThreadState* newState){
        m.goTo(newState);
    }
};

class Init : MasterThreadState {
public:
    static MasterThreadState& State() { return instance; }
    virtual void recieved(MasterThread& master, int tag);
private:
    static Init instance;
};

class Idle : MasterThreadState {
public:
    void recieved(MasterThread& master,int tag);
    static MasterThreadState& State(){ return instance; }
private:
    static Idle instance;
};

错误是：

嵌套名称说明符中使用了不完整类型 'Init' _state = Init::State();

Answer 1

虚函数没有实现

这段代码工作正常：

class MasterThreadState;
class Init; //class Idle;

class MasterThread
{
public:
    MasterThread();
    void handle(int sender, int tag);
private:
    int _sender;
    MasterThreadState* _state;
private:
    friend class MasterThreadState;
    void goTo(MasterThreadState* newState){ _state = newState; }
};

class MasterThreadState
{
public:
    virtual void recieved(MasterThread& master, int tag) = 0;
protected:
    void goTo(MasterThread& m, MasterThreadState* newState){
        m.goTo(newState);
    }
};

class Init : MasterThreadState {
public:
    static MasterThreadState* State();
    virtual void recieved(MasterThread& master, int tag);
private:
    Init(){}
};

class Idle : MasterThreadState {
public:
    static MasterThreadState* State();
    virtual void recieved(MasterThread& master,int tag);
private:
    static Idle instance;
};

Answer 2

如果在 Init 声明之后移动 MasterThread 构造函数，代码将编译（并构建）：

class MasterThread
{
public:
    MasterThread();
    void handle(int sender, int tag);
private:
    int _sender;
    MasterThreadState* _state;
private:
    friend class MasterThreadState;
    void goTo(MasterThreadState* newState){ _state = newState; }
};

class MasterThreadState
{
public:
    virtual void recieved(MasterThread& master, int tag) {}
protected:
    void goTo(MasterThread& m, MasterThreadState* newState){
        m.goTo(newState);
    }
};

class Init : MasterThreadState {
public:
    static MasterThreadState* State() { return &instance; }
    virtual void recieved(MasterThread& master, int tag) {}
private:
    static Init instance;
};

Init Init::instance;

MasterThread::MasterThread() {
    _state = Init::State();
}

注意，Init::State() 应该 return 指针，MasterThreadState*。这也发生了变化，定义了静态成员 Init::instance，并将虚函数的实现（空）正确添加到 link。

C++ 状态模式异常

C++ State pattern exception

c++

design-patterns

state-pattern