Re: [Boost-users] [attn. Dave Abrahams] workaround ideas?

Subject: Re: [Boost-users] [attn. Dave Abrahams] workaround ideas?
From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2009-03-04 03:53:33

On Tue, Mar 3, 2009 at 11:27 PM, Ovanes Markarian <om_boost_at_[hidden]>wrote:

> Noah,
>
> please see my answer below:
>
> On Tue, Mar 3, 2009 at 5:59 PM, Noah Roberts <roberts.noah_at_[hidden]>wrote:
>
>>
>> template < typename FIELD, typename CDR >
>> typename FIELD::type & get(rec< FIELD, CDR > & r)
>> {
>>  return r.value;
>> }
>
>
>  You should use the static_cast<SomeFieldSpecificType>(r) here to fix the
> problem. The only problem is, that 'r' is the entire recordset, and you
> can't cast it to itself, nor you can extract the SomeFieldSpecificType.
>
> Your example work with MSVC 9. I have rewritten it to use the static_cast,
> but did not test it with MSVC 8. Give it a try, I think this should work:
>
>
> #include <boost/mpl/fold.hpp>
> #include <boost/mpl/vector.hpp>
> #include <boost/mpl/inherit.hpp>
> #include <boost/mpl/identity.hpp>
> #include <boost/mpl/placeholders.hpp>
> #include <boost/mpl/inherit_linearly.hpp>
>
>
>
> namespace pl = boost::mpl::placeholders;
> namespace mpl = boost::mpl;
>
>
> struct empty {};
>
> template <typename FIELD>
> struct rec
> {
>     typename FIELD::type value;
>
>     rec() : value() {}
> };
>
> template < typename VEC >
> struct generate_record
>     : boost::mpl::inherit_linearly< VEC, mpl::inherit< rec<pl::_2>, pl::_1>
> >::type
> {};
>
>
>
>
> template < typename FIELD, class Records>
> typename FIELD::type & get(Records& rs)
> {
>     return static_cast<rec<FIELD>&>(rs).value;
> }
>
>
> typedef mpl::identity<int>    field1;
> typedef mpl::identity<double> field2;
> typedef mpl::identity<char> field3;
> typedef mpl::identity<int> field4;
>
> typedef generate_record<mpl::vector< field1, field2, field3> >::type
> test_record;
>
> int main()
> {
>     test_record tr;
>     get<field1>(tr) = 3;
>     get<field2>(tr) = 3.0;
>     get<field3>(tr) = 'a';
>     get<field4>(tr) = 10; //this won't work as intended
>
>     return 0;
> }
>
> This might be interesting to Dave:
> If same types are passed into the inheritance, no compiler error is issued
> since the types are probably managed by mpl::set(???). Compiler will
> silently compile but get<field4>(tr) = 10 will overwrite the value of
> get<field1>(tr), which is probably not intended. And we intend in MPL to
> produce as mucht errors as possible, since the compiler is our tester ;)
>
> I would expect, that at least here an error is produced, that I am trying
> to derive from the same type twice. This is a philosofical discussion, since
> I can modify inherit to inherit virtually and this might be fine than, but
> throwing away the type from the inheritance is not ok. At least MSVC
> produced the following type:
>
> boost::mpl::inherit2<rec<boost::mpl::identity<char>
> >,boost::mpl::inherit2<rec<boost::mpl::identity<double>
> >,rec<boost::mpl::identity<int> > > >
>

Sorry for the last misleading paragraph. I forgot to put the field4 into the
typesequence. After putting it in I received expected compiler error. MPL
Rocks!

Best Regards,
Ovanes