Re: [Boost-users] [mpl] insert not inserting?

From: David Abrahams (dave_at_[hidden])
Date: 2007-03-29 22:06:13

on Thu Mar 29 2007, Scott Meyers <usenet-AT-aristeia.com> wrote:

> David Abrahams wrote:
>> :) Not much of an advance.  Try "studying my detailed elaboration" and
>> let me know if that's still the case.
>
> Things are better, thanks primarily to Joaquín M López Muñoz's helpful 
> explanation.  Part of my confusion, I think, can be traced to the design 
> decision behind this comment of yours:
>
>> mpl::set<...> may actually have a ::type
>> member (essentially a typedef for mpl::set<...> itself), but if so
>> it's just there as a convenience for use with constructs like eval_if.
>
> ...
>
>> There's a class of metafunctions that usually don't need to be
>> explicitly invoked: those whose range of results is limited in certain
>> ways.  In practice this turns out to mean metafunctions with
>> numeric/boolean results.  
>
> The convenience you write of is real, but it also means that some metafunction 
> invocations need no ::type, but others do.  Without understanding the rules in 
> some detail, it's hard to figure out when ::type is required.  The MPL design 
> chooses convenience over consistency, and at the beginning of the learning 
> curve, my personal opinion is that consistency is more important than 
> convenience.  For experienced users, of course, the balance point may be -- 
> probably is -- at a different location.

You're still free to use all the unnecessary typename ... ::type
incantations if you want to be consistent and safe.

Vesa Karvonen's "Lazy MPL," which I mentioned in an earlier posting,
would probably be much easier overall because it only needs ::type in
a very few well-defined places.

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