From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-27 18:48:48
Peter Dimov wrote:
> Daniel Walker wrote:
>> _1 == _2 // ambiguous
>> lambda::hold(_1) == _2 // lambda expression
>> lambda::release(_1) == _2 // 3rd-party expression
> I think that the right thing here (if we take the interoperability issue to
> heart) is to make
> _1 == _2
> "just work". It's an interesting challenge, but it's not impossible since
> that is exactly what is_bind_expression is intended to solve.
> The next milestone, boost::bind( f, _1 + _2 ), is harder since it requires
> boost::bind to respect specializations of is_bind_expression.
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?
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.
-- Eric Niebler Boost Consulting www.boost-consulting.com
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