c++状态模式异常

C++ State pattern exception

本文关键字：异常 模式 状态 c++      更新时间：2023-10-16

我是c++新手!请帮助我实现国家模式。

我有下面的代码，但它不能编译:

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了不完整类型'Init'

虚函数未实现

这段代码工作得很好:

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;
};

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

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()应该返回指针MasterThreadState*。这也被改变了，定义了静态成员Init::instance，并添加了虚函数的实现(空)以正确链接。

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