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

From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-27 19:32:49

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 only minor complication is that for the placeholders and
>> expressions containing them, operator() must evaluate the expression
>> rather than create a larger expression template. But that's trivial
>> to do with proto.
> 
> Trivial? :-) Are you assuming variadic templates 

No.

and decltype?

No, but I *am* assuming Boost.Typeof.

Consider _9 -
> _4 as one simple example.
> 
> Either way, if proto does evaluate, there would be no need for most of 
> Lambda. 

Precisely. :-)

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