boost/iterator/is_lvalue_iterator.hpp

   1 // Copyright David Abrahams 2003. Use, modification and distribution is
   2 // subject to the Boost Software License, Version 1.0. (See accompanying
   3 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   4 #ifndef IS_LVALUE_ITERATOR_DWA2003112_HPP
   5 # define IS_LVALUE_ITERATOR_DWA2003112_HPP
   6
   7 #include <boost/iterator.hpp>
   8
   9 #include <boost/detail/workaround.hpp>
  10 #include <boost/detail/iterator.hpp>
  11
  12 #include <boost/iterator/detail/any_conversion_eater.hpp>
  13
  14 // should be the last #includes
  15 #include <boost/type_traits/detail/bool_trait_def.hpp>
  16 #include <boost/iterator/detail/config_def.hpp>
  17
  18 #ifndef BOOST_NO_IS_CONVERTIBLE
  19
  20 namespace boost {
  21
  22 namespace detail
  23 {
  24 #ifndef BOOST_NO_LVALUE_RETURN_DETECTION
  25   // Calling lvalue_preserver( <expression>, 0 ) returns a reference
  26   // to the expression's result if <expression> is an lvalue, or
  27   // not_an_lvalue() otherwise.
  28   struct not_an_lvalue {};
  29
  30   template <class T>
  31   T& lvalue_preserver(T&, int);
  32
  33   template <class U>
  34   not_an_lvalue lvalue_preserver(U const&, ...);
  35
  36 # define BOOST_LVALUE_PRESERVER(expr) detail::lvalue_preserver(expr,0)
  37
  38 #else
  39
  40 # define BOOST_LVALUE_PRESERVER(expr) expr
  41
  42 #endif
  43
  44   // Guts of is_lvalue_iterator.  Value is the iterator's value_type
  45   // and the result is computed in the nested rebind template.
  46   template <class Value>
  47   struct is_lvalue_iterator_impl
  48   {
  49       // Eat implicit conversions so we don't report true for things
  50       // convertible to Value const&
  51       struct conversion_eater
  52       {
  53           conversion_eater(Value&);
  54       };
  55
  56       static char tester(conversion_eater, int);
  57       static char (& tester(any_conversion_eater, ...) )[2];
  58
  59       template <class It>
  60       struct rebind
  61       {
  62           static It& x;
  63
  64           BOOST_STATIC_CONSTANT(
  65               bool
  66             , value = (
  67                 sizeof(
  68                     is_lvalue_iterator_impl<Value>::tester(
  69                         BOOST_LVALUE_PRESERVER(*x), 0
  70                     )
  71                 ) == 1
  72             )
  73           );
  74       };
  75   };
  76
  77 #undef BOOST_LVALUE_PRESERVER
  78
  79   //
  80   // void specializations to handle std input and output iterators
  81   //
  82   template <>
  83   struct is_lvalue_iterator_impl<void>
  84   {
  85       template <class It>
  86       struct rebind : boost::mpl::false_
  87       {};
  88   };
  89
  90 #ifndef BOOST_NO_CV_VOID_SPECIALIZATIONS
  91   template <>
  92   struct is_lvalue_iterator_impl<const void>
  93   {
  94       template <class It>
  95       struct rebind : boost::mpl::false_
  96       {};
  97   };
  98
  99   template <>
 100   struct is_lvalue_iterator_impl<volatile void>
 101   {
 102       template <class It>
 103       struct rebind : boost::mpl::false_
 104       {};
 105   };
 106
 107   template <>
 108   struct is_lvalue_iterator_impl<const volatile void>
 109   {
 110       template <class It>
 111       struct rebind : boost::mpl::false_
 112       {};
 113   };
 114 #endif
 115
 116   //
 117   // This level of dispatching is required for Borland.  We might save
 118   // an instantiation by removing it for others.
 119   //
 120   template <class It>
 121   struct is_readable_lvalue_iterator_impl
 122     : is_lvalue_iterator_impl<
 123           BOOST_DEDUCED_TYPENAME boost::detail::iterator_traits<It>::value_type const
 124       >::template rebind<It>
 125   {};
 126
 127   template <class It>
 128   struct is_non_const_lvalue_iterator_impl
 129     : is_lvalue_iterator_impl<
 130           BOOST_DEDUCED_TYPENAME boost::detail::iterator_traits<It>::value_type
 131       >::template rebind<It>
 132   {};
 133 } // namespace detail
 134
 135 // Define the trait with full mpl lambda capability and various broken
 136 // compiler workarounds
 137 BOOST_TT_AUX_BOOL_TRAIT_DEF1(
 138     is_lvalue_iterator,T,::boost::detail::is_readable_lvalue_iterator_impl<T>::value)
 139
 140 BOOST_TT_AUX_BOOL_TRAIT_DEF1(
 141     is_non_const_lvalue_iterator,T,::boost::detail::is_non_const_lvalue_iterator_impl<T>::value)
 142
 143 } // namespace boost
 144
 145 #endif
 146
 147 #include <boost/iterator/detail/config_undef.hpp>
 148 #include <boost/type_traits/detail/bool_trait_undef.hpp>
 149
 150 #endif // IS_LVALUE_ITERATOR_DWA2003112_HPP