On 08/28/12 07:22, Larry Evans wrote: [snip]
Now, the following questions concern the test code (compute_bases.cpp) attached to my previous post. The output of that test code shows that one of the superclasses of any<requirements_t> is:
concept_interface < requirements_t . . .
However, I can't think of a use case for providing such a superclass. The unspecialized concept_interface definition:
template<class Concept, class Base, class ID, class Enable = void> struct concept_interface : Base {};
has an empty body; hence, the only use of putting such a concept_interface into one of the superclasses of an any (as opposed to just using Base) is to allow for a specialization of it. That's completely unexpected to me, and I suspect, to others. Also, the Concepts in Depth tutorial:
http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/...
provides no examples where the 1st template argument to concept_interface is an mpl sequence of other concepts, leading the casual reader to jump to the conclusion that the Concept arg to concept_interface is *not* an mpl sequence.
Could you please provide a use case for such a concept_interface specialization so users could understand when it might be useful?
[snip] iterator.hpp provides such a use case: template<class T, class Reference, class DifferenceType, class ValueType> struct iterator< ::boost::no_traversal_tag, T, Reference, DifferenceType, ValueType> : boost::mpl::vector< copy_constructible<T>, constructible<T()>, equality_comparable<T>, dereferenceable<typename iterator_reference<Reference, ValueType>::type, T>, assignable<T> > { typedef ValueType value_type; typedef typename iterator_reference<Reference, ValueType>::type reference; typedef DifferenceType difference_type; }; template<class T, class Reference, class DifferenceType, class ValueType> struct iterator< ::boost::incrementable_traversal_tag, T, Reference, DifferenceType, ValueType> : boost::mpl::vector< iterator< ::boost::no_traversal_tag, T, Reference, DifferenceType>, incrementable<T> > { typedef ValueType value_type; typedef typename iterator_reference<Reference, ValueType>::type reference; typedef DifferenceType difference_type; }; template<class T, class Reference, class DifferenceType, class ValueType> struct iterator< ::boost::forward_traversal_tag, T, Reference, DifferenceType, ValueType> : iterator< ::boost::incrementable_traversal_tag, T, Reference, DifferenceType, ValueType> {}; template< class T = _self, class Reference = boost::use_default, class DifferenceType = std::ptrdiff_t
struct forward_iterator : iterator<boost::forward_traversal_tag, T, Reference, DifferenceType> {};