Re: [boost] [serialization] Inherited objects

From: Simon Buchan (simon_at_[hidden])
Date: 2005-10-11 16:56:11

David Abrahams wrote:
> "Robert Ramey" <ramey_at_[hidden]> writes:
> 
> 
>>Simon Buchan wrote:
>>
>>>Robert Ramey wrote:
>>>
>>>>Double check that a is polymorphic - that is, that it has at least
>>>>one virtual function.  Then get back to us.
>>>>
>>>>Robert Ramey
>>>>
>>>>>I'm guessing that is supposed to be:
>>>>>
>>>>>main(){
>>>>>   ...
>>>>>   ar.register_type<derived_one>();
>>>>>   ar.register_type<derived_two>();
>>>>>   base *b;
>>>>>   ar & b;
>>>>>}
>>>>
>>>>
>>>>nope, its correct.  This is a little known quirk of C++ syntax.
>>>>Your second version won't compile on the most conforming compilers.
>>>>
>>>Not according to Comeau:
>>>"ComeauTest.c", line 16: error: the "template" keyword used for
>>>          syntactic disambiguation may only be used within a template
>>>
Quick note: Comeau (possibly the worlds most conforming compiler) 
doesn't actually require the template disambiguator (despite allowing it 
in the correct case), when I asked them why 
(support_at_[hidden]), they replied with:

" We believe there is enough wiggle room and amibiguity in the standard
" about that requirement, and also, some direction that if the rule does
" exist, that it might be relaxed.

So basically, they don't think it's _really_ that required by the 
standard. (Personally, I believe at least a warning in strict mode would 
be nice)

>>whoops - wrong again.  If ar is a template you need "ar.template" other wise 
>>you shouldn't have it.  
> 
> Nope, wrong again.  If ar's type X is dependent and "register_type" is a
> nested template in X, then you need "ar.template register_type<...>".
> Otherwise, you need "ar.register_type<...>"
> 
A-la typename usage:

        typename dependant_name::nested_type

        dependant_name.template nested_template

>>From the above code it could be either.  Its really annoying to me
>>to have to keep the context and provinence of a variable like "ar"
>>in my head while I'm writting.  
> 
> Guess what; you don't.  If you had to know whether ar was a template
> or not, just think how impossible it would be to write generic code?
> 
Well, you *do* have to know if ar is a class template, but not if it's a 
template class, which is what I think you were trying to say.