Re: Understanding MPL

From: David Abrahams (dave_at_[hidden])
Date: 2003-03-31 19:18:57

"Joel de Guzman" <djowel_at_[hidden]> writes:

> Edward Diener wrote:
>> David Abrahams wrote:
>>> "Edward Diener" <eddielee_at_[hidden]> writes:
>
>>> There's an unspecified type-expression which goes where the
>>> /*unspecified*/ comment is.
>>> 
>>>> I always though that a typedef must specify an actual type but I
>>>> don't see any in the expression above.
>>> 
>>> The point is that we're not showing it to you ;-)
>> 
>> I suspected that but I couldn't figure out why you wouldn't show it.
>> However it made it hard for me to understand how mpl::if_<> really
>> works without understanding what actually goes there.
>
> Because it's an implementation detail :-) You need not know
> the guts of how if_ really works, as long as it works. The same
> way as you don't need to know how if (c) a; else b; works, 
> (in terms of low level code) as long as it works.
>
> However, perhaps the /*unspecified*/ would better be enhanced
> by pseudo-code? 
>
>     template<
>         typename Condition
>         , typename T1
>         , typename T2
>         >
>     struct if_
>     {
>         /* if (Condition) T1 else T2 */
>         typedef /* implementation detail */ type;
>     };
>
> Thoughts? Dave? Aleksey?

We're not hacking the paper at this point (working on the book instead
;-))

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com