Re: [Boost-users] Understanding MPL and using '::type'

From: Edward Diener (eldiener_at_[hidden])
Date: 2008-05-07 09:03:01

Ovanes Markarian wrote:
> Edward,
> 
> please see below.
> 
> On Wed, May 7, 2008 at 5:30 AM, Edward Diener <eldiener_at_[hidden] 
> <mailto:eldiener_at_[hidden]>> wrote:
> 
>     Ovanes Markarian wrote:
>      > Edward,
>      >
>      > Just wanted to add my 2 cent. That might help. You need ::type
>     when you
>      > do type calculations.
>      > For example:
>      >
>      > Access the type stored in a type sequence -> ::type
> 
>     OK
> 
>      > transform type sequence to another type sequence -> ::type
> 
>     OK
> 
>      > So meta-algorithms operate on types and result in new types -> ::type
> 
>     There is nothing in the book or documentation reference about
>     "meta-algorithms", whatever they are.
> 
> The authors of the book make an analogy between STL and MPL. Especially 
> by introducing meta-functions which are algorithms.
> Take a look at chapter 6.
> 6.1. Algorithms, Idioms, Reuse, and Abstraction
> 6.2. Algorithms in MPL
> ...
> 
> MPL meta-functions like transform, sort, copy, remove, reverse, replace 
> etc. are analogies to STL algorithms.

I understand what the algorithms of MPL are. You are saying that one 
always appends "::type" to the result of calling one of the MPL 
algorithms. That is one rule, which Steve also had already mentioned in 
a previous reply.

So now I have some rules garnered from your and Steve's replies about 
when to append ::type when using MPL constructs.

Thanks for your help.

I still find the irregularity of when to use '::type' and when not to 
use '::type' the most difficult part of using MPL but at least I am 
further along than before.