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

Subject: Re: [Boost-users] reference to '_1' is ambiguous
From: Eric Niebler (eric_at_[hidden])
Date: 2009-03-18 18:07:56

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.

> 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.

> Our tests don't show the problem since they don't use precompiled 
> headers. 

Step 1: add tests that exhibit the problem with precompiled headers.

> So we've relied on bug reports to iron these out, leading to 
> the current status quo in boost/bind/placeholders.hpp.
> 
> Using inline functions:
> 
>    static inline boost::arg<1> _1() { return boost::arg<1>(); }
> 
> is the most conservative option since it works everywhere, but it does 
> have the disadvantage that one cannot overload operators for _1.

And bind is one of those libraries that we want to work even on ancient 
compilers.... <sigh> Maybe we can have a separate Boost.Bind v2 library, 
like we now have signals and signals2.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com