Re: [Boost-Users] Re: Understanding MPL

From: Joel de Guzman (djowel_at_[hidden])
Date: 2003-03-31 18:43:17

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?

-- 
Joel de Guzman
joel at boost-consulting.com
http://www.boost-consulting.com
http://spirit.sf.net