[Boost-users] Re: mpl -> integral wrappers sequences -> enum types ?

Martin Pasdzierny writes:

Stepping forward in dealing with a mpl::vector_c of enums I found it hard to determine the correct expression to 'mpl::find' an element within such a vector. The code below compiles with gcc 3.3.3 and shows 5 different approaches, but only version 4,5 really find the correct iterator. Version 1,2,3 return iterators pointing to the vectors end. It would be helpful if someone more familiar with MPL's concepts could give some explanation, particularly about the aspects:

- Is there a solution using mpl::find instead of mpl::find_if

Yes (assuming that the sequence you are searching in satisfies Integral Sequence Wrapper requirements): typedef vector_c v; typedef find< v, integral_c >::type iter; ^^^

If not, why the difference between finding elements in mpl::vector and mpl::vector_c ?

'find' uses *type equality* ('boost::is_same') to determine whether the elements are the same, and from standpoint of the C++ compiler, all these integral_c integral_c int_<c> have different types, so you have to be precise in specifying what exactly you are looking for. The 'find' snippet above is guaranteed to work because all Integral Sequence Wrappers classes are required to be wrappers around seqn,integral_c, ... integral_c > (see http://www.boost.org/libs/mpl/doc/refmanual/integral-sequence-wrapper.html). Note that unless you have control over how your sequences of integral constants are constructed, the approach is fragile: typedef vector_c v1; typedef find< v1, integral_c >::type iter1; // OK typedef push_back< my_vector_c, int_<Z> >::type v2; // ^^^^^^^ typedef find< v1, integral_c >::type iter2; // Nothing found! The current recommendation for generic code working with integral constants is to always use explicit 'equal_to' comparison. If you find this too verbose, you might consider writing a helper metafunction that would wrap this up for you, e.g.: template< typename Seq, typename Seq::value_type N > struct find_c : find_if< Seq, equal_to<_1, integral_c > { }; typedef find_c< v1, Z >::type iter2; // Works

- What is the difference between using integral wrappers: boost::mpl::integral_c > boost::mpl::int_<c>

See the above.

- And still the question from the previous posting: Can I (should I) tell mpl::vector_c I'm using my own enum type and not plain int ?

Not at the moment, see my earlier reply. HTH, -- Aleksey Gurtovoy MetaCommunications Engineering