C++ C++；：从外部在类内声明枚举数，以便可以在私有成员中使用

不能在类之外声明嵌套枚举。在使用它之前，还必须定义它

class Printer;
enum Printer::States;
class Printer {                  // choose one of monitor or cormonitor
    States taskStates[];
  public:
    enum States { Starting = 'S', Blocked = 'B', Unblocked = 'U', Finished = 'F', // general
          Napping = 'N', Awake = 'A',             // Santa
          Working = 'W', NeedHelp = 'H',          // elf
          OnVacation = 'V', CheckingIn = 'I',     // reindeer
          DeliveringToys = 'D', DoneDelivering = 'd', // Santa, reindeer
          Consulting = 'C', DoneConsulting = 'c' // Santa, elves
    };
    Printer();
    void print( unsigned int id, States state );
    void print( unsigned int id, States state, unsigned int numBlocked );
};

---

作为旁注，C++11的

enum类

只需在类内声明-它可以在类外定义。

不能在类外声明嵌套的枚举。在使用它之前，还必须定义它

class Printer;
enum Printer::States;
class Printer {                  // choose one of monitor or cormonitor
    States taskStates[];
  public:
    enum States { Starting = 'S', Blocked = 'B', Unblocked = 'U', Finished = 'F', // general
          Napping = 'N', Awake = 'A',             // Santa
          Working = 'W', NeedHelp = 'H',          // elf
          OnVacation = 'V', CheckingIn = 'I',     // reindeer
          DeliveringToys = 'D', DoneDelivering = 'd', // Santa, reindeer
          Consulting = 'C', DoneConsulting = 'c' // Santa, elves
    };
    Printer();
    void print( unsigned int id, States state );
    void print( unsigned int id, States state, unsigned int numBlocked );
};

---

作为旁注，C++11的

enum类

只需在类内声明-它可以在类外定义。

看起来您正在尝试向前声明枚举。在C++03中，这是非法的。在C++11中，只要指定枚举的基础类型，就可以执行此操作。来自维基百科：

因此，如果编译器支持前向声明的枚举，则可以启用C++0x/C++11支持，将代码更改为：

enum Enum1;                      // Illegal in C++03 and C++11; the underlying type cannot be determined.
enum Enum2 : unsigned int;       // Legal in C++11, the underlying type is explicitly specified.
enum class Enum3;                // Legal in C++11, the underlying type is int.
enum class Enum4 : unsigned int; // Legal C++11.
enum Enum2 : unsigned short;     // Illegal in C++11, because Enum2 was previously declared with a different underlying type.

如果不是，则无法将枚举范围限定到类。您可以创建一个单独的命名空间，并使用typedef获得类似的语法：

class Printer;
enum Printer::States : char;
class Printer {                  // choose one of monitor or cormonitor
    States taskStates[];
  public:
    enum States : char { Starting = 'S', Blocked = 'B', Unblocked = 'U', Finished = 'F', // general
          Napping = 'N', Awake = 'A',             // Santa
          Working = 'W', NeedHelp = 'H',          // elf
          OnVacation = 'V', CheckingIn = 'I',     // reindeer
          DeliveringToys = 'D', DoneDelivering = 'd', // Santa, reindeer
          Consulting = 'C', DoneConsulting = 'c' // Santa, elves
    };
    Printer();
    void print( unsigned int id, States state );
    void print( unsigned int id, States state, unsigned int numBlocked );
};

然后在Printer类之外，在看到Printer类定义之前，需要将状态称为Printer:：State，而不是Printer:：State。在查看了Printer类定义之后，可以像往常一样将状态引用为Printer:：States（因为typedef）

---

或者，如果删除名称空间，您只需将它们称为状态。

看起来您正试图向前声明枚举。在C++03中，这是非法的。在C++11中，只要指定枚举的基础类型，就可以执行此操作。来自维基百科：

因此，如果编译器支持前向声明的枚举，则可以启用C++0x/C++11支持，将代码更改为：

enum Enum1;                      // Illegal in C++03 and C++11; the underlying type cannot be determined.
enum Enum2 : unsigned int;       // Legal in C++11, the underlying type is explicitly specified.
enum class Enum3;                // Legal in C++11, the underlying type is int.
enum class Enum4 : unsigned int; // Legal C++11.
enum Enum2 : unsigned short;     // Illegal in C++11, because Enum2 was previously declared with a different underlying type.

如果不是，则无法将枚举范围限定到类。您可以创建一个单独的命名空间，并使用typedef获得类似的语法：

class Printer;
enum Printer::States : char;
class Printer {                  // choose one of monitor or cormonitor
    States taskStates[];
  public:
    enum States : char { Starting = 'S', Blocked = 'B', Unblocked = 'U', Finished = 'F', // general
          Napping = 'N', Awake = 'A',             // Santa
          Working = 'W', NeedHelp = 'H',          // elf
          OnVacation = 'V', CheckingIn = 'I',     // reindeer
          DeliveringToys = 'D', DoneDelivering = 'd', // Santa, reindeer
          Consulting = 'C', DoneConsulting = 'c' // Santa, elves
    };
    Printer();
    void print( unsigned int id, States state );
    void print( unsigned int id, States state, unsigned int numBlocked );
};

然后在Printer类之外，在看到Printer类定义之前，需要将状态称为Printer:：State，而不是Printer:：State。在查看了Printer类定义之后，可以像往常一样将状态引用为Printer:：States（因为typedef）

---

或者，如果您删除名称空间，您只需将它们引用为状态。

或者您可以在声明

枚举后创建一个私有部分，并将其放入其中……您是在询问如何/在何处定义
状态
？还是在声明
枚举后创建一个私有部分并将其放入其中……您是在询问如何/在何处要定义
状态
？