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 15:25:23

Peter Dimov wrote:
> Eric Niebler:
>>
>> You, Joel de Guzman, Dan Marsden and I should coordinate. We might be 
>> able to use Proto so that the bind placeholders can be shared with the 
>> Phoenix placeholders, eliminating this source of confusion once and 
>> for all.
> 
> Bind can already use any placeholders (and in fact does recognize the 
> Lambda placeholders). No coordination is needed, just a specialization 
> of boost::is_placeholder.
> 
> The main problem is that all code that relies on the placeholders being 
> visible will break. 

You mean code that requires the placeholders to be at global scope? Why 
not bring them into the global scope with a using declaration?

> A secondary problem I have is that to keep Bind 
> header-only I (think I) need to move to using inline functions instead 
> of objects for _K on all compilers. This will consume one release cycle, 
> during which the global placeholders will be "deprecated". The next 
> release will remove them from the global namespace.

You are talking about ODR violations in code that refers to const 
objects at namespace scope? There are ways around this problem....

namespace boost { namespace placeholders {

template<class I>
struct placeholder_instance
{
   placeholder<I> const value;
};

// statically initialized ...
template<class I>
placeholder<I> const placeholder_instance<I>::value = {{}};

namespace
{
   // This reference should also be statically initialized
   placeholder<mpl::int_<1> > const & _1 =
     placeholder_instance<mpl::int_<1> >::value;
}

}}

// for back-compatibility ...
using boost::placeholders::_1;

Now every use of _1 refers to the same object, even across translation 
units, and it's all header-only. Not all compilers support this yet, but 
Dave and I fought to get this approved for C++0x. For older compilers, I 
think we can live with the ODR, as these objects will be stateless 
(identity doesn't matter).

If placeholder<> is an appropriately-defined Proto terminal, then it can 
be reused in Phoenix.

> But if there's a Boost-wide header that defines _K, I can easily make 
> Bind use it.

I think it should all be possible.

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