traits.qbk

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[section:traits Iterator Traits]

`std::iterator_traits` provides access to five associated types
of any iterator: its `value_type`, `reference`, `pointer`,
`iterator_category`, and `difference_type`.  Unfortunately,
such a "multi-valued" traits template can be difficult to use in a
metaprogramming context.  `<boost/iterator/iterator_traits.hpp>`
provides access to these types using a standard metafunctions_.

[h2 Synopsis]

Header `<boost/iterator/iterator_traits.hpp>`:

  template <class Iterator>
  struct iterator_value
  {
      typedef typename 
        std::iterator_traits<Iterator>::value_type 
      type;
  };

  template <class Iterator>
  struct iterator_reference
  {
      typedef typename 
        std::iterator_traits<Iterator>::reference
      type;
  };

  template <class Iterator>
  struct iterator_pointer
  {
      typedef typename 
        std::iterator_traits<Iterator>::pointer 
      type;
  };

  template <class Iterator>
  struct iterator_difference
  {
      typedef typename
        detail::iterator_traits<Iterator>::difference_type
      type;
  };

  template <class Iterator>
  struct iterator_category
  {
      typedef typename
        detail::iterator_traits<Iterator>::iterator_category
      type;
  };

[h2 Broken Compiler Notes]

Because of workarounds in Boost, you may find that these
[@../../mpl/doc/index.html#metafunctions metafunctions] actually work better than the facilities provided by
your compiler's standard library.

On compilers that don't support partial specialization, such as
Microsoft Visual C++ 6.0 or 7.0, you may need to manually invoke
[@../../type_traits/index.html#transformations BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION] on the
`value_type` of pointers that are passed to these metafunctions.

Because of bugs in the implementation of GCC-2.9x, the name of
`iterator_category` is changed to `iterator_category_` on that
compiler.  A macro, `BOOST_ITERATOR_CATEGORY`, that expands to
either `iterator_category` or `iterator_category_`, as
appropriate to the platform, is provided for portability.

[endsect]