Is empty enum ( enum{}; ) portable?

Question

class Test
{
  enum{};
  ...
};

Is this empty enum definition portable? Compiles in gcc and and msvc.

Answer 1

According to the following snippet from the c++ standard we can deduce that it's indeed a valid statement:

7.2/1 Enumeration declarations (C++03)
...
enum-specifier:
       enum identifier_opt { enumerator-list_opt }

Note that both the identifier and the enumerator-list are optional, and therefor a statement as enum {} is valid (if you ask the standard).

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But doesn't the standard also say that empty declarations are ill-formed?

Yes, and there is even an example of enum { }; in the below snippet from the standard.

7/3 Specifiers (C++03)

In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in these specifiers are among the names being declared by the declaration (as class- names, enum-names, or enumerators, depending on the syntax).

In such cases, and except for the declaration of an unnamed bit-field (9.6), the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration.

*Example [

 enum { };          // ill-formed
 typedef class { }; // ill-formed

*end example]

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Conclusion

The statement seems to be ill-formed after a careful look at the standard, though compilers are written by humans - and humans tend to make mistakes and sometimes overlook things.

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TL;DR You should not use an empty declaration such as enum { };, even though it compiles

Answer 2

such an enum is specifically listed in clause 7 paragraph 3 of the C++ standard as ill-formed. gcc does not accept it. there was a bug fix for this in gcc:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29018