Boost logo

Boost :

From: David Abrahams (dave_at_[hidden])
Date: 2005-10-11 15:39:55

"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
> 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<...>"

> 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?

Dave Abrahams
Boost Consulting

Boost list run by bdawes at, gregod at, cpdaniel at, john at