specialize a member template without specializing its parent

Question

I have a class template nested inside another template. Partially specializing it is easy: I just declare another template< … > block inside its parent.

Answer 1

Complex stuff. Your initial code ICE's VC10 Beta2, nice.

First off, I think you have this backwards:

template<> 
template< class X > 
struct A< X >::B< int > {};

X is a template param to struct A, and B is the one fully specialized, so I think it should be this:

template< class X > 
template<> 
struct A< X >::B< int > {};

But even this fails to compile. The error text is actually useful, though:

a.cpp a.cpp(11) : error C3212: 'A::B' : an explicit specialization of a template member must be a member of an explicit specialization a.cpp(8) : see declaration of 'A::B'

It looks like it is only legal to fully specialize B if you also fully specialize A.

Edit: Ok, I heard back from someone who can speak authoritatively on this - to paraphrase, this is a very murky area in the standard, and it's an open issue with the C++ Committee to clean it up ("it" being explicit specializations of members of class templates). In the near term, the advice is "Don't do that".