是类的枚举转发声明吗？ [英] Is in-class enum forward declaration possible?

问题描述

我知道在C ++ 11可以转发声明一个枚举类型（如果提供存储类型）。

 枚举E：short; 
 void foo（E e）; 
 
 .... 
 
枚举E：short 
 {
 VALUE_1，
 VALUE_2，
 .... $但我想转发声明一个类中定义的枚举，例如
 
 枚举Foo :: E：short; 
 void foo（E e）; 
 
 .... 
 
 class Foo 
 {
 enum E：short 
 {
 VALUE_1，
 VALUE_2，
 .... 
} 
} 
  
这样的可能在C ++ 11？
解决方案
不，这样的向前声明是不可能的。 [decl.enum] / 5（粗体强调我）：
 
 
如果 enum-key   
 枚举指示符应指的是以前在类中直接声明的枚举，即  
  nested-name-specifier 所指向的命名空间（即既不是继承的，也不是使用声明引入的
） 
 
 
（在这种情况下， nested-name-specifier 将是您的类的名称，后跟 :: 。）
 
但是，枚举外部并使用不透明枚举声明。
 
I know that in C++11 it's possible to forward declare an enum type (if storage type is provided) e.g. 
enum E : short;
void foo(E e);

....

enum E : short
{
    VALUE_1,
    VALUE_2,
    ....
}
But I would like to forward declare an enum defined within a class e.g.
enum Foo::E : short;
void foo(E e);

....

class Foo
{
    enum E : short
    {
        VALUE_1,
        VALUE_2,
    ....
    }
}
Is something like this possible in C++11 ?
解决方案
No, such a forward declaration isn't possible. [decl.enum]/5 (bold emphasis mine):

  
If the enum-key is followed by a nested-name-specifier, the
  enum-specifier shall refer to an enumeration that was previously
  declared directly in the class or namespace to which the
  nested-name-specifier refers (i.e., neither inherited nor introduced
  by a using-declaration), and the enum-specifier shall appear in a
  namespace enclosing the previous declaration.
(In this case the nested-name-specifier would be the name of your class followed by a ::.)

You could, though, put the enumeration outside and use an opaque-enum-declaration.

                        
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