[boost] [typeof] Determining if a type has been registered or not at compile time

Subject: [boost] [typeof] Determining if a type has been registered or not at compile time
From: Edward Diener (eldiener_at_[hidden])
Date: 2009-12-22 09:47:49

Is there any way one can determine whether a type has been registered or 
not with Boost.Typeof at compile time so that the information can be 
used in TMP ? I am thinking, of course, of the use of typeof in a 
template, where a template parameter may or may not be aregistered type 
when the template has been instantiated. I would like to provide my own 
TMP fallback code if the type has not been registered rather than  see 
an immediate compiler error generated. Something like:

boost::mpl::eval_if
   <
   boost::typeof::registered<T>, // this would be nice to have
   boost::mpl::identity<BOOST_TYPEOF_TPL(some_expression)>,
   boost::mpl::identity</* use another facility to get the type I want 
into a metafunction*/>
   >