Is it possible to specialize a template definition based on the existence of a nested typedef of a template type parameter?

Question

I have a template, template class wrapper, that I would like to specialize based on the existence of typename T::context_type. If

Answer 1

Yes, it is possible. I have implemented such behavior in the past by using some metaprogramming tricks. The basic build blocks are:

BOOST_MPL_HAS_XXX_TRAIT_DEF, to define a metafunction predicate that will evaluate to a true type if the argument is of class type and has a nested type with a given name (context_type in your case).

http://www.boost.org/doc/libs/1_47_0/libs/mpl/doc/refmanual/has-xxx-trait-def.html

Boost.EnableIf, to define the specializations based on the previously defined trait.

http://www.boost.org/libs/utility/enable_if.html # See 3.1 Enabling template class specializations

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Note that you may be able to get that behavior working directly with SFINAE, something like this may work:

template< typename T, typename Context = void >
class wrapper { ... }; // Base definition

template< typename T >
class wrapper< T, typename voif_mfn< typename T::context_type >::type > { ... }; // Specialization

However, I like the expressiveness of the solution based on traits and enable if.