Re: [boost] [type_traits] is_base_of<B, D> should work when B is incomplete

Subject: Re: [boost] [type_traits] is_base_of<B, D> should work when B is incomplete
From: John Maddock (jz.maddock_at_[hidden])
Date: 2016-03-04 04:24:42

>> I have an incomplete type boost::in_place_factory. I want to check if a
>> given type T is either derived from boost::in_place_factory. It doesn't
>> have to be very strict: it can get false negative on multiple inheritance;
>> it can give a false positive when T is implicitly convertible to
>> boost::in_place_factory by other means, or simply return false_type when
>> boost::in_place_factory is incomplete.
>>
>> Perhaps there is a C++03-compatible solution to this particular problem?
>>
> Actually, I do not even need a true-false answer; I just need to disable an
> overload (in SFINAE sense) if boost::in_place_factory is either incomplete
> or not a base class of T.

This may be fixable (or alternatively the fix may just break a ton of 
things.... we'll see), but I'm curious why in_place_factory needs to be 
incomplete - surely if you handle that base class a special case then 
you need it's definition anyway?  In any case why not just include the 
header and be done with?

Thanks, John.