阅读下面UML类图，给出C++形式的头文件

答案：

class CallBase;

struct ICallStateListener

{

    virtual void OnStateChange(CallBase* call) = 0;

};

enum CallStatus

{

    Ringing,

    Connecting,

    Connected,

    Disconnecting,

    Disconnected

};

class CallBase

{

public:

    const string Number;

public:

    void AddListener(ICallStateListener* lsn);

    void RemoveListener(ICallStateListener* lsn);

    virtual void Hangup();

    virtual void SetStatus(CallStatus status);

    CallStatus GetStatus()const;

    int GetDuration();

protected:

    void SetNumber(const string& number);

};

class InCall: public CallBase

{

public:

    void Accept();

};

class OutCall: public CallBase

{

public:

    void Call(const string& number);

    void SendDTMF(char key);

};

class CallManager

{

public:

    void OnDialKey();

    void OnCancelKey();

    void OnHangupKey();

    void OnFlashKey();

private:

    CallBase* _calls[2];

};​​​​