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

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 >::type > { static typename Field::type apply(Params & params) { return get<Field>(params); } };

template < typename FieldValue, typename Params > FieldValue init_field(Params & pars) { return initializer::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 // > ... // };

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?