[boost] Re: [MPL] Lambda

From: David Abrahams (dave_at_[hidden])
Date: 2004-05-14 15:20:43

Just to clarify, we're talking about mpl::lambda here.

"David B. Held" <dheld_at_[hidden]> writes:

> Ok, suppose we have a policy-based class Foo, and a policy P:
>
> template <typename Policy>
> class Foo
> {
>     // ...
> };
>
> struct P
> {
>     template <typename T>
>     struct apply
>     {
>         typedef policy type;
>     };
> };

P is a "metafunction class"

> Now, my understanding is that we can use Lambda so that we can
> invoke Foo this way:
>
> Foo<P> f;

for some definition of "invoke" ;-)

> Or define P in this way:
>
> template <typename T>
> struct P
> {
>     // policy
> };

P is a "metafunction"

> and call Foo this way:

For some defintion of "call" ;-)

> Foo<P<_> > f;

P<_> is a "lambda expression"

> But if I require that everyone define P in the first way, then Lambda
> should not be necessary, is that correct?  Is there still a reason to
> use Lambda in that case?

In the upcoming version of MPL (very soon now) mpl::apply -- which
you'll want to use even to invoke metafunction classes portably (no,
using P::template apply<whatever>::type doesn't work everywhere) --
will invoke mpl::lambda internally, so the upshot is that passing
either lambda expressions or metafunction classes will work.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com