Re: [Boost-users] reference to '_1' is ambiguous

Subject: Re: [Boost-users] reference to '_1' is ambiguous
From: Peter Dimov (pdimov_at_[hidden])
Date: 2009-03-18 18:36:33

Eric Niebler:
> Peter Dimov wrote:
>> Eric Niebler:
>> ...
>>> namespace
>>> {
>>>   // This reference should also be statically initialized
>>>   placeholder<mpl::int_<1> > const & _1 =
>>>     placeholder_instance<mpl::int_<1> >::value;
>>> }
>>
>> This is a problem for some precompiled header implementations.
>
> Huh.

Yep. Apparently, the unnamed namespace does not get an unique name in every 
translation unit because of the precompilation.

>> Some compilers have fixed theirs to allow it, some have not, some only 
>> allow(ed?) it when the variables are static.
>
> I notice I left "static" out of the example I sent around, but yes, 
> placeholder_instance<>::value is supposed to be a class static.

This is not what I meant. On some compilers (going by the #ifdefs, MSVC and 
Metrowerks), this:

namespace
{
    X x;
}

doesn't work in a precompiled header, but this:

namespace
{
    static X x;
}

does (because a static x doesn't cause a multiple definition link error.)

On Borland, neither works in a PCH. On g++ 4, both work but the second 
causes an "unused variable" warning for every _K that is not referenced, 
which is a bit annoying. I'm not sure what works on g++ 3, but it uses the 
inline function path. :-)