Re: [Boost-users] [MPL] initialization of member that might be non-copyable

Subject: Re: [Boost-users] [MPL] initialization of member that might be non-copyable
From: Larry Evans (cppljevans_at_[hidden])
Date: 2011-03-26 00:17:29

On 03/25/11 15:53, Noah Roberts wrote:
> On 3/25/2011 1:48 PM, Noah Roberts wrote:
> [snip bad code]
>
> I posted bits that I'd been experimenting and reverted stuff without
> testing.  Here's corrected version:
>
>
> template < typename Field >
> struct field_value
> {
>   typedef Field field;
>   typedef typename Field::type ftype;
>   ftype value;
>
>   field_value(ftype const& t) : value(t) {}
>   field_value() : value() {}
> };
>
> template < typename Field, typename Params, typename Enable = void >
> struct initializer
> {
>   static typename Field::type apply(Params &)
>   {
>     return Field::type();
>   }
> };
> template < typename Field, typename Params >
> struct initializer<Field,Params, typename boost::enable_if<
> boost::mpl::contains<typename Params::fields, Field> >::type >
> {
>   static typename Field::type apply(Params & params)
>   {
>     return get<Field>(params);
>   }
> };
>
> template < typename FieldValue, typename Params >
> FieldValue init_field(Params & pars)
> {
>   return initializer<typename FieldValue::field, Params >::apply(pars);
> }
>
>
> template < typename FieldValue, typename More >
> struct record_base : FieldValue, More
> {
>   template < typename Params >
>   record_base(Params & pars)
>     : FieldValue(init_field<FieldValue>(pars))
>     , More(pars)
>   {}
>
>   record_base() : FieldValue(), More() {}
> };
> struct empty_record_base
> {
>   template < typename Ignored >
>   empty_record_base(Ignored&){}
>   empty_record_base() {}
> };
>
> template < typename MplSequence >
> struct build_record
> {
>   typedef boost::mpl::placeholders::_1 _1;
>   typedef boost::mpl::placeholders::_2 _2;
>
>   typedef typename boost::mpl::fold
>     <
>       MplSequence
>     , empty_record_base
>     , record_base< field_value<_2>, _1 >
>     >::type type;
>
>   // type should be record_base< field_value<field0>
>   //                           , record_base< field_value<field1>
>   //                           , ...
>   //                           , record_base<field_value<fieldN>
>   //                           , empty_record_base>
>   //                           > ...
>   //
> };

Hi Noah,

After comparing with Section 9.5 of the MPL book, it seems the
correspondence between this code and the book's is:

    this_code                     the book
    ---------                     ----------
    empty_record_base             empty_base
    field_value                   wrap
    record_base                   inherit

and the major difference is is field_value, which, I suppose will be
used (after possibly some modification) to allow a non-copyable
member. Is that right?  Since record_base has no constructor taking
this into account, I'm guessing you're really asking how to create
such a constructor and maybe modifications to field_value to make this
possible.  Right?

-regards,
Larry

BTW, there's no get template defined above.  I assume it will
be something like the get on p. 192 of the book?