Question

我是c ++新手！请帮我实施国家模式。

我有下面的代码，但它没有编译：

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：

在嵌套名称说明符中使用的不完整类型“Init” _state = Init :: State（）;

Answer 1

虚拟功能尚未实施

此代码工作正常：

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（和，build）：

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（）应返回指针MasterThreadState*。这也被改变了，静态成员Init :: instance定义，虚拟函数的实现（空）被添加到链接中。

C ++状态模式异常

2 个答案: