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

7 May 2008

      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.
...
You don't need ::type when you pass parameters or transport the 
contained type(s), e.g.
What do you mean by the above ?
...
Placeholder transports type parameter to a metafunction -> no ::type
OK.
...
vector of type passed to a metafunction also transports type(s) -> no ::type
OK
...
Hope that helps.
It helps a little but it does not break the two general cases in my mind 
into all the specific instances of using mpl constructs in the reference.

I am gathering that the correct specific way to determine whether or not 
to append "::type" is to look at the mpl construct and if it is a 
metafunction, which has its own nested type, always append "::type". 
More general rules regarding categories of decision about appending 
"::type", as you have suggested above, seem very vague to me except in 
specific cases which Steve and you have mentioned.

Eddie
...
Ovanes
P.S. sorry that I did not remove unrelated posting lines.
On Tue, May 6, 2008 at 9:44 PM, Steven Watanabe mailto:watanabesj@gmail.com> wrote:
AMDG
Edward Diener wrote:
     > I understand the explanation about how placeholders work. The
    rule you
     > are specifying appears to be that one never adds the '::type' to
    the end
     > of a placeholder expression when it is being passed to an mpl
    construct.
     > Is that correct ?
     >
You should never add ::type to a placeholder expression at all.
    Otherwise, the metafunction will probably get very confused.
> I wish I knew what the is an mpl function as opposed to what is
    not an
     > mpl function. Perhaps the mpl algorithms are the mpl functions and
     > everything which is not an algorithm is not an mpl function.
     >
Inserters, Sequences, and the types listed under "Data Types" are not
    metafunctions.
    Everything listed in a section with Metafunctions of Algorithms in its
    name are metafunctions.
>>  If your not
     >> sure, the
     >> only thing to do is look it up.
     >>
     >
     > Look up what ? Where in the reference documentation does it tell me
     > whether I need to add '::type' to the end of an mpl construct ?
     >
Here are two examples from the reference:
template< typename Sequence , typename State , typename ForwardOp >
    struct accumulate <./accumulate.html> {
       typedef /unspecified/ type;
    };
template< typename Seq >
    struct back_inserter <./back-inserter.html> {
       // /unspecified/
       // /.../
    };
In Christ,
    Steven Watanabe
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