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

7 May 2008

      Ovanes Markarian wrote:
...
Edward,
please see below.
On Wed, May 7, 2008 at 5:30 AM, Edward Diener mailto:eldiener@tropicsoft.com> 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.