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.