Re: [boost] Re: additional test for is_abstract type trait

From: John Maddock (john_at_[hidden])
Date: 2004-11-30 05:49:13

> John, now that you're at is_abstract, I wonder whether you could
> consider adding some workaround in is_abstract.hpp making
> boost::is_abstract return unconditionally false for those compilers
> known not to work for it.
> You can read about the origin of this request at:
>
> http://aspn.activestate.com/ASPN/Mail/Message/boost/2209389
>
> In a nutshell, Boost.Serialization does some (ODR-nonconformant)
> redefinition of boost::is_abstract the way I'm suggesting. These
> precautions are necessary because, on those compilers not
> supporting the is_abstract implementation, the construct can crash
> instead of returning false, and this makes it impossible to use
> the facility without carefully taking care of the compiler.

Uh, I hadn't realised that that was the case.  However, here's the question: 
is it always true that returning false is always safe?  If the user's code 
uses is_abstract to determine whether an actual instance of a class can 
exist, then won't it do the wrong thing?  I can (just about) understand 
is_polymorphic having a safe default, but is that true of is_abstract?

I suppose what I'm saying is that the users idea of what constitutes a safe 
default may vary from use case to use case.

We should define an appropriate macro though, whatever else is decided.

John.