Re: [Boost-users] Multiple templates singletons

Ovanes Markarian wrote:

Hello!

I have a question to this construct:

template class TreeImplBase;

Each specialization of these holds a static std::map< boost::tuple, const TreeImplBase* >; which contains the list of the derived template classes.

How can you instantiate a pointer to the class temlate? Class template is an incomplete type. You will not be able to specify a pointer to it.

The wording is a bit ambiguous: A pointer to an /incomplete type/ is in fact OK (using standard terminology), but 'TreeImplBase' is a /template id/ which is not a type at all. As Ovanes pointed out that construct is illegal, however (except inside a definition of 'TreeImplBase' where it's the /injected class name/). /Specializing/ a template with template arguments yields a type. Using (whatever) member of this type's interface causes the template to be implicitly (possibly partially) /instantiated/. It's most important to realize that specializing a template does not cause the template to be instantiated automatically. Note that "template specialization" has a context-dependent meaning, as it can either refer to a type or of a class (template) definition of a variant implementation of the template: // /primary template/ template< typename T > struct A { ... }; // definition of the (full) specialization A<int> template< > struct A<int> { ... }; // definition of a partial specialization template< typename T > struct A< B<T> > { typedef A self; // /injected class name/ is a type }; // X and Y are specializations (not instantiations)! typedef A< B<int> > X; typedef A< long > Y; int main() { X x; // instantiation of the 'B<int>'-specialization of A // at this point // ... OK, that's about template terminology in five minutes :-). Regards, Tobias