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Have you ever wanted to write a generic function that can operate on any kind of dereferenceable object? If you have, you've probably run into the problem of how to determine the type that the object "points at":
template <class Dereferenceable> void f(Dereferenceable p) { *what-goes-here?* value = \*p; ... }
pointee
        
          It turns out to be impossible to come up with a fully-general algorithm
          to do determine what-goes-here directly,
          but it is possible to require that pointee<Dereferenceable>::type
          is correct. Naturally, pointee
          has the same difficulty: it can't determine the appropriate ::type
          reliably for all Dereferenceables,
          but it makes very good guesses (it works for all pointers, standard and
          boost smart pointers, and iterators), and when it guesses wrongly, it can
          be specialized as necessary:
        
namespace boost { template <class T> struct pointee<third_party_lib::smart_pointer<T> > { typedef T type; }; }
indirect_reference
        
          indirect_reference<T>::type is rather more specialized than
          pointee, and is meant to
          be used to forward the result of dereferencing an object of its argument
          type. Most dereferenceable types just return a reference to their pointee,
          but some return proxy references or return the pointee by value. When that
          information is needed, call on indirect_reference.
        
          Both of these templates are essential to the correct functioning of indirect_iterator.
        
minimum_category
        
          minimum_category takes
          two iterator categories or two iterator traversal tags and returns the
          one that is the weakest (i.e. least advanced). For example:
        
static_assert( is_same< minimum_category< std::forward_iterator_tag, std::random_access_iterator_tag >::type, std::forward_iterator_tag >::value, "Unexpected minimum_category result" );
The library provides several utilities to simplify conversions between iterator categories and traversal tags:
iterator_category_to_traversal<C>::type
              - the metafunction takes an iterator category C
              and returns the corresponding traversal tag.
            iterator_traversal<T>::type
              - a shorthand for iterator_category_to_traversal<iterator_category<T>::type>::type.
            pure_traversal_tag<T>::type
              - the metafunction takes a tag T
              which derives from one of the iterator traversal tags and returns that
              traversal tag. T may
              also derive from other tags describing the iterator (e.g. whether this
              is a const-iterator or
              not), these additional tags are not considered.
            pure_iterator_traversal<T>::type
              - a shorthand for pure_traversal_tag<iterator_traversal<T>::type>::type.
            pointee
        template <class Dereferenceable> struct pointee { typedef /* see below */ type; };
          Requires: For an object x of type Dereferenceable,
          *x
          is well-formed. If ++x
          is ill-formed it shall neither be ambiguous nor shall it violate access
          control, and Dereferenceable::element_type
          shall be an accessible type. Otherwise iterator_traits<Dereferenceable>::value_type
          shall be well formed. [Note: These requirements need not apply to explicit
          or partial specializations of pointee]
        
          type is determined according
          to the following algorithm, where x
          is an object of type Dereferenceable:
        
if ( ++x is ill-formed ) { return Dereferenceable::element_type } else if (*x is a mutable reference to std::iterator_traits<Dereferenceable>::value_type) { return iterator_traits<Dereferenceable>::value_type } else { return iterator_traits<Dereferenceable>::value_type const }
indirect_reference
        template <class Dereferenceable> struct indirect_reference { typedef /* see below */ type; };
          Requires: For an object x of type Dereferenceable,
          *x
          is well-formed. If ++x
          is ill-formed it shall neither be ambiguous nor shall it violate access
          control, and pointee<Dereferenceable>::type&
          shall be well-formed. Otherwise iterator_traits<Dereferenceable>::reference
          shall be well formed. [Note: These requirements need not apply to explicit
          or partial specializations of indirect_reference]
        
          type is determined according
          to the following algorithm, where x
          is an object of type Dereferenceable:
        
if ( ++x is ill-formed ) return pointee<Dereferenceable>::type& else std::iterator_traits<Dereferenceable>::reference
minimum_category
        template <typename C1, typename C2> struct minimum_category { typedef /* see below */ type; };
          Requires: Both C1
          and C2 shall be standard
          iterator categories or iterator traversal tags.
        
          type is determined according
          to the following algorithm, where c1
          is an object of type C1
          and c2 is an object of
          type C2:
        
if (c1 is convertible to c2) return C2; else return C1;
| ![[Note]](../../../../../../doc/src/images/note.png) | Note | 
|---|---|
| The above definition relies on the fact that the more restricting categories and traversal tags are convertible to the less restricting ones. | 
iterator_category_to_traversal
        template <typename C> struct iterator_category_to_traversal { typedef /* see below */ type; };
          Requires: C
          shall be a standard iterator category or an iterator traversal tag.
        
          If C is an iterator traversal
          tag or convertible to one, type
          equivalent to C. Otherwise,
          type is defined to the
          closest iterator traversal tag matching C.
        
iterator_traversal
        template <typename Iterator> struct iterator_traversal { typedef typename iterator_category_to_traversal< typename iterator_category<Iterator>::type >::type type; };
          Requires: Iterator
          shall be an iterator.
        
pure_traversal_tag
        template <typename T> struct pure_traversal_tag { typedef /* see below */ type; };
          Requires: T
          shall be convertible to an iterator traversal tag.
        
          type is defined to be the
          most advanced traversal tag Tag
          so that T is convertible
          to Tag.
        
pure_iterator_traversal
        template <typename Iterator> struct pure_iterator_traversal { typedef typename pure_traversal_tag< typename iterator_traversal<Iterator>::type >::type type; };
          Requires: Iterator
          shall be an iterator.