boost/detail/identifier.hpp

0001 //  boost/identifier.hpp  ----------------------------------------------------//
0002
0003 //  Copyright Beman Dawes 2006
0004
0005 //  Distributed under the Boost Software License, Version 1.0. (See accompanying
0006 //  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
0007
0008 //  See documentation at http://www.boost.org/libs/utility
0009
0010 #ifndef BOOST_IDENTIFIER_HPP
0011 #define BOOST_IDENTIFIER_HPP
0012
0013 #include <boost/utility/enable_if.hpp>
0014 #include <boost/type_traits/is_base_of.hpp>
0015 #include <iosfwd>
0016
0017 namespace boost
0018 {
0019   namespace detail
0020   {
0021     //  class template identifier  ---------------------------------------------//
0022
0023     //  Always used as a base class so that different instantiations result in
0024     //  different class types even if instantiated with the same value type T.
0025
0026     //  Expected usage is that T is often an integer type, best passed by
0027     //  value. There is no reason why T can't be a possibly larger class such as
0028     //  std::string, best passed by const reference.
0029
0030     //  This implementation uses pass by value, based on expected common uses.
0031
0032     template <typename T, typename D>
0033     class identifier
0034     {
0035     public:
0036       typedef T value_type;
0037
0038       const value_type value() const           { return m_value; }
0039       void  assign( value_type v )             { m_value = v; }
0040
0041       bool operator==( const D & rhs ) const   { return m_value == rhs.m_value; }
0042       bool operator!=( const D & rhs ) const   { return m_value != rhs.m_value; }
0043       bool operator< ( const D & rhs ) const   { return m_value <  rhs.m_value; }
0044       bool operator<=( const D & rhs ) const   { return m_value <= rhs.m_value; }
0045       bool operator> ( const D & rhs ) const   { return m_value >  rhs.m_value; }
0046       bool operator>=( const D & rhs ) const   { return m_value >= rhs.m_value; }
0047
0048       typedef void (*unspecified_bool_type)(D); // without the D, unspecified_bool_type
0049       static void unspecified_bool_true(D){}    // conversion allows relational operators
0050                                                 // between different identifier types
0051
0052       operator unspecified_bool_type() const   { return m_value == value_type() ? 0 : unspecified_bool_true; }
0053       bool operator!() const                   { return m_value == value_type(); }
0054
0055     // constructors are protected so that class can only be used as a base class
0056     protected:
0057       identifier()                             {}
0058       explicit identifier( value_type v )      : m_value(v) {}
0059
0060     private:
0061       T m_value;
0062     };
0063
0064   //#ifndef BOOST_NO_SFINAE
0065
0066   //  template <class Ostream, class Id>
0067   //    typename enable_if< is_base_of< identifier< typename Id::value_type, Id >, Id >,
0068   //      Ostream & >::type operator<<( Ostream & os, const Id & id )
0069   //  {
0070   //    return os << id.value();
0071   //  }
0072
0073   //  template <class Istream, class Id>
0074   //    typename enable_if< is_base_of< identifier< typename Id::value_type, Id >, Id >,
0075   //      Istream & >::type operator>>( Istream & is, Id & id )
0076   //  {
0077   //    typename Id::value_type v;
0078   //    is >> v;
0079   //    id.value( v );
0080   //    return is;
0081   //  }
0082   //#endif
0083
0084   } // namespace detail
0085 } // namespace boost
0086
0087 #endif // BOOST_IDENTIFIER_HPP