http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49896
--- Comment #2 from Matthieu Imbert <matthieu.imb...@ens-lyon.fr> 2011-07-29 10:37:33 UTC --- (In reply to comment #1) > You lack a definition of test<C>::version. C++ standard section 9.4.2 states: If a 'static' data member is of 'const' integral or 'const' enumeral type, its declaration in the class definition can specify a 'constant-initializer' which shall be an integral constant expression (5.19). In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program and the namespace scope definition shall not contain an 'initializer'. The notion of "used" is defined in section 3.2: An object or non-overloaded function whose name appears as a potentially- evaluated expression is used unless it is an object that satisfies the requirements for appearing in a constant expression (5.19 [expr.const]) and the lvalue-to-rvalue conversion (4.1 [conv.lval]) is immediately applied. I am not a C++ expert but various sources (for example: stroustrup C++ faq: http://www2.research.att.com/~bs/bs_faq2.html#in-class) seem to say that the code snippet i sent is valid since the address of test<C>::version is not taken. Stroustrup says: "You can take the address of a static member if (and only if) it has an out-of-class definition", which i interpret as: no need for a definition if address not taken. So in brief, i think there is no need to define test<C>::version to be correct.
