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

From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2008-05-06 13:40:22

AMDG

Greetings from BoostCon,

Edward Diener wrote:
> template<class Seq>
> struct biggest_float_as_double :
>    mpl::deref
>    <
>      typename mpl::max_element
>      <
>        typename mpl::replace
>        <
>          Seq,
>          float,
>          double
>        >::type,
>        mpl::less
>        <
>          mpl::sizeof_<_1>,
>          mpl::sizeof_<_2>
>        >
>      >::type
>    >
> {};
>
> In two cases in the nested construct above one uses the 'typename 
> xxx<...>::type' construct whereas in one case one does not. There seems 
> to be some logic or rule about this but I do not understand what it is.
>   

The ::type is no needed on the deref because we want biggest_float_as_double
to inherit type nested ::type of deref.  Thus, 
biggest_float_as_double<S>::type will
be deref<...>::type.  This technique is known as metafunction forwarding.

> A few pages later in the text I see:
>
> typedef mpl::vector
> <
>    mpl::vector_c<int, 1, 2, 3>,
>    mpl::vector_c<int, 4, 5, 6>,
>    mpl::vector_c<int, 7, 8, 9>
>  >
> S;
>
> typedef mpl::transform
> <
>    S,
>    mpl::front<_>,
>    mpl::inserter
>    <
>      mpl::int_<0>,
>      mpl::plus<_,_>
>    >
>  >
> ::type sum;
>
> Here we have another nested mpl construct and I do understand the logic 
> of it, but here there are no internal nested '::type' statements. Once 
> again I am utterly puzzled about the whys and wherefores of using or not 
> using '::type'.
>   

front<_> is an MPL lambda expression.  It is not supposed to
be evaluated immediately.  mpl::transform will evaluate it substituting
each element of the sequence for _.  leaving the ::type of plus<...>
is a shortcut which works for numeric metafunctions.  There would
be no problem with adding a ::type.

mpl::inserter is an mpl object rather than an mpl metafunction.

> Finally I turn the page and I see this MPL construct:
>
> template<class Seq>
> struct reverse :
>    mpl::fold
>    <
>      Seq,
>      typename mpl::clear<Seq>::type,
>      mpl::push_front<_,_>
>    >
> {};
>
> Once again I can follow the logic and the explanation of mpl::fold, but 
> I have no idea by what rule one of the nested constructs has '::type' 
> appended and the next one does not.

Again,

mpl::push_front<_,_>

is a lambda expression.  Its evaluation is delayed.

In Christ,
Steven Watanabe