[boost] Re: Non-standard feature proposed in help text of operators library??

From: Daniel Frey (daniel.frey_at_[hidden])
Date: 2003-07-23 04:15:59

David Abrahams wrote:
> You're right.

Only WRT some limited points, but probably not in general :)

> It works because the base classes of T are
> "associated" in:
> 
>      struct myclass : X<myclass> {};

I think the OP asked about explicit instantiated X<myclass>. Daniel 
(Spangenberg), please correct me if I'm wrong, but you question boils 
down to something like this:

namespace A
{
    template< typename T > struct X
    {
       friend void operator+(T,T) {}
    };
}

namespace B
{
    struct Y {};
    template struct A::X< Y >; // explicit instantiation
}

int main()
{
    B::Y a;
    B::Y b;

    a + b; // <-- ??
}

The above code compiles fine with the GCC 2.95.2. The GCC 3.3 doesn't 
find the operator (it injects it into A, not into B), the Comeau rejects 
the explicit instantiation. Initially I thought that the above example 
should be legal, as the standard reads in 14.6.5/1:

"When a template is instantiated, the names of its friends are treated 
as if the specialization had been explicitly declared at its point of 
instantia-tion."

The point of instantiation is in namespace B, but after thinking about 
it a bit longer and reading some more stuff from the standard, I wonder 
if this can be true. The instantiation could happen in a different 
translation unit, so it seems strange that the operator is found in the 
same translation unit but not in different ones. Also, the Comeau 
rejects the explicit instantiation as it happens inside the namespace. 
14.7.2 is also pretty clear on the subject, so I think that the GCC 3.3 
is correct, the Comeau should probably turn the error into a warning as 
the instantiation is not illegal (AFAICS, but correct me if I'm wrong), 
but probably has surprising behaviour. The GCC 2.95.2 and the operators 
library is wrong, the OP's observation is correct that it uses 
non-standard code. Also, I need to understand why my reading of 14.6.5/1 
is wrong, or is this a DR? Should I take it to csc++?

Regards, Daniel

-- 
Daniel Frey
aixigo AG - financial training, research and technology
Schloß-Rahe-Straße 15, 52072 Aachen, Germany
fon: [phone], fax: [phone]
eMail: daniel.frey_at_[hidden], web: http://www.aixigo.de