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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.