C++：从外部声明类内的枚举器，以便可以在私有成员中使用

Question

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 );
};

Answer 1

您不能在类之外声明嵌套枚举。您还必须在使用它之前对其进行定义。

class Printer {                  // choose one of monitor or cormonitor
  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
    };
  private:
    States taskStates[];
  public:
    Printer();
    void print( unsigned int id, States state );
    void print( unsigned int id, States state, unsigned int numBlocked );
};

顺便说一句，C++11 的 enum class 只需在类内部声明 - 它可以在类外部定义。

Answer 2

看起来您正在尝试前向声明一个枚举。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做到这一点。来自维基百科： http://en.wikipedia.org/wiki/C%2B% 2B11#Strongly_typed_enumerations

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.

因此，如果您的编译器支持前向声明枚举，您可以打开 C++0x/C++11 支持更改您的代码：

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 );
};

如果不是，您就无法将枚举范围限定为该类。您可以创建一个单独的命名空间并使用 typedef 来获得类似的语法：

class Printer;

namespace printer {
    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
    };
}

class Printer {                  // choose one of monitor or cormonitor
    States taskStates[];
  public:
    typedef printer::States States;

    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，您需要将 States 称为 Printer::States 而不是 Printer::州。了解 Printer 类定义后，您可以像往常一样将状态称为 Printer::States（因为 typedef）。

或者，如果您删除名称空间，您只需将它们称为状态。