boost/detail/identifier.hpp

   1 //  boost/identifier.hpp  ----------------------------------------------------//
   2
   3 //  Copyright Beman Dawes 2006
   4
   5 //  Distributed under the Boost Software License, Version 1.0. (See accompanying
   6 //  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   7
   8 //  See documentation at http://www.boost.org/libs/utility
   9
  10 #ifndef BOOST_IDENTIFIER_HPP
  11 #define BOOST_IDENTIFIER_HPP
  12
  13 #include <boost/utility/enable_if.hpp>
  14 #include <boost/type_traits/is_base_of.hpp>
  15 #include <iosfwd>
  16
  17 namespace boost
  18 {
  19   namespace detail
  20   {
  21     //  class template identifier  ---------------------------------------------//
  22
  23     //  Always used as a base class so that different instantiations result in
  24     //  different class types even if instantiated with the same value type T.
  25
  26     //  Expected usage is that T is often an integer type, best passed by
  27     //  value. There is no reason why T can't be a possibly larger class such as
  28     //  std::string, best passed by const reference.
  29
  30     //  This implementation uses pass by value, based on expected common uses.
  31
  32     template <typename T, typename D>
  33     class identifier
  34     {
  35     public:
  36       typedef T value_type;
  37
  38       const value_type value() const           { return m_value; }
  39       void  assign( value_type v )             { m_value = v; }
  40
  41       bool operator==( const D & rhs ) const   { return m_value == rhs.m_value; }
  42       bool operator!=( const D & rhs ) const   { return m_value != rhs.m_value; }
  43       bool operator< ( const D & rhs ) const   { return m_value <  rhs.m_value; }
  44       bool operator<=( const D & rhs ) const   { return m_value <= rhs.m_value; }
  45       bool operator> ( const D & rhs ) const   { return m_value >  rhs.m_value; }
  46       bool operator>=( const D & rhs ) const   { return m_value >= rhs.m_value; }
  47
  48       typedef void (*unspecified_bool_type)(D); // without the D, unspecified_bool_type
  49       static void unspecified_bool_true(D){}    // conversion allows relational operators
  50                                                 // between different identifier types
  51
  52       operator unspecified_bool_type() const   { return m_value == value_type() ? 0 : unspecified_bool_true; }
  53       bool operator!() const                   { return m_value == value_type(); }
  54
  55     // constructors are protected so that class can only be used as a base class
  56     protected:
  57       identifier()                             {}
  58       explicit identifier( value_type v )      : m_value(v) {}
  59
  60   #if !defined(BOOST_MSVC) || BOOST_MSVC > 1300  // 1300 == VC++ 7.0 bug workaround
  61     private:
  62   #endif
  63       T m_value;
  64     };
  65
  66   //#ifndef BOOST_NO_SFINAE
  67
  68   //  template <class Ostream, class Id>
  69   //    typename enable_if< is_base_of< identifier< typename Id::value_type, Id >, Id >,
  70   //      Ostream & >::type operator<<( Ostream & os, const Id & id )
  71   //  {
  72   //    return os << id.value();
  73   //  }
  74
  75   //  template <class Istream, class Id>
  76   //    typename enable_if< is_base_of< identifier< typename Id::value_type, Id >, Id >,
  77   //      Istream & >::type operator>>( Istream & is, Id & id )
  78   //  {
  79   //    typename Id::value_type v;
  80   //    is >> v;
  81   //    id.value( v );
  82   //    return is;
  83   //  }
  84   //#endif
  85
  86   } // namespace detail
  87 } // namespace boost
  88
  89 #endif // BOOST_IDENTIFIER_HPP