RE: [boost] Re: MPL not_ etc. questions

From: Brock Peabody (brock.peabody_at_[hidden])
Date: 2003-09-05 15:25:55

> -----Original Message-----
> From: boost-bounces_at_[hidden] [mailto:boost-bounces_at_[hidden]]
> On Behalf Of David Abrahams
> Sent: Friday, September 05, 2003 2:09 PM
> To: boost_at_[hidden]
> Subject: [boost] Re: MPL not_ etc. questions

[...]

> The other advantage to using mpl::not_<X> rather than
> mpl::not_<X>::type is that it can delay the instantiation of X when X
> is a template specialization:
> 
>       mpl::apply_if<
>           expr1
>         , mpl::not_<complicated_bool_metafunction<X> >
>         , mpl::true_
>       >::type z;
> 
> if expr1 turns out to be mpl::false_, the complicated_metafunction
> will never be instantiated.

Am I right in betting that this would short-circuit and compile:

   typename mpl::and_<
      expr1<X>,
      expr_that_is_only_valid_if_expr1_is<X>
   >::type

Where:

   mpl::bool_<
      expr1<X>::value
      && expr_that_is_only_valid_if_expr1_is<X>::value
   >

Would not?

Brock