Re: [boost] [bind][tr1] patch for more compliant placeholders

From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-27 20:16:20

Peter Dimov wrote:
> Eric Niebler wrote:
>> Peter Dimov wrote:
>>> Eric Niebler wrote:
>>>
>>>> Although I don't know all the details of bind and lambda, I had
>>>> hoped that _1 and _2 would be abstract proto entities, such that
>>>>
>>>>    _1 == _2
>>>>
>>>> creates an expression template like:
>>>>
>>>>    equal< terminal< arg<1> >, terminal< arg<2> > >
>>>>
>>>> which is a generic, abstract representation of an equals expression
>>>> with two placeholders. Can't bind and lambda both be implemented to
>>>> recognize such a type and do the appropriate thing with it?
>>> Possibly, if they are made to honor is_bind_expression
>>> specializations and result_of, if is_bind_expression reports true
>>> for the above type, if the above is a function object that actually
>>> returns x == y, and if result_of works for it.
>> Yes, that can all be made to work without much difficulty.
> 
> The part about boost::bind using is_bind_expression and result_of cannot, 
> not without major surgery and possibly compromising portability.

I'll have to take your word for it -- I'm not familiar enough with the 
TR1 bind to appreciate the difficulty of respecting is_bind_expression. 
It seems to me that is_bind_expression can be trivially implemented, 
given appropriately defined placeholders. Eg. if they are defined in a 
super-secret hidden namespace, like so:

namespace super_secret_hidden {
   template<class I> struct arg { typedef I arity; };
   template<class T> yes_type is_bind_expression_impl(T const *);
}
no_type is_bind_expression_impl(void const *);

Now _1 is:

proto::terminal<super_secret_hidden::arg<mpl::int_<1> > >::type const _1 
= {{}};

And is_bind_expression<> is trivially implementable using ADL and 
sizeof. Should work for older compilers, too, right? Are their compilers 
we support that don't do ADL?

Result_of support for older compilers ... I suppose that requires 
partial template specialization, right? Do we still support compilers 
that don't do PTS?

> Sometimes I get excited about the new C++0x features and tell myself, why 
> not rewrite boost::bind to take advantage of them? Then it occurs to me that 
> C++0x has a std::bind built-in. :-) (Happened twice so far.) 

Haha!

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com