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

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-04-27 19:11:36

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.

> 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 and decltype? Consider _9 - 
_4 as one simple example.

Either way, if proto does evaluate, there would be no need for most of 
Lambda.