[boost] Re: Request for enable_if.hpp

From: Jonathan D. Turkanis (technews_at_[hidden])
Date: 2003-12-05 16:03:00

"Jaakko Jarvi" <jajarvi_at_[hidden]> wrote in message
news:Pine.LNX.4.53.0312051058570.9894_at_damogran.osl.iu.edu...
 > > Can we add conditional preprocessor guards around the contents of the
 > > enable_if.hpp header so that the templates are only defined on
 > > supporting compilers?
 > >
 >
 > Ok, I'm guarding it with BOOST_NO_SFINAE, then any use of enable_if on
 > a compiler which does not support SFINAE is an error.
 > This is the safe way, (compared to providing dummy definitions that
 > do not give the desired behavior).
 >
 >   Jaakko

This could lead to very uninformative compiler errors if someone tries 
to use enable_if on an unsupported compiler. I tried it on VC6 and found 
that if enable_if is used without explicit namespace qualification I 
don't get a single error which clearly states that enable_if is not defined.

This might have been a reason to write enable_if without partial
specialization, even though no compiler supports SFINAE but not partial
spec. A static assert could inform users that SFINAE is not available.

The dummies with static asserts might be a good alternative, to avoid
complicating the main definitions.

Jonathan