boost/blank.hpp

   1 //-----------------------------------------------------------------------------
   2 // boost blank.hpp header file
   3 // See http://www.boost.org for updates, documentation, and revision history.
   4 //-----------------------------------------------------------------------------
   5 //
   6 // Copyright (c) 2003
   7 // Eric Friedman
   8 //
   9 // Distributed under the Boost Software License, Version 1.0. (See
  10 // accompanying file LICENSE_1_0.txt or copy at
  11 // http://www.boost.org/LICENSE_1_0.txt)
  12
  13 #ifndef BOOST_BLANK_HPP
  14 #define BOOST_BLANK_HPP
  15
  16 #include "boost/blank_fwd.hpp"
  17
  18 #include <iosfwd> // for std::basic_ostream forward declare
  19
  20 #include "boost/detail/templated_streams.hpp"
  21 #include "boost/mpl/bool.hpp"
  22 #include "boost/type_traits/is_empty.hpp"
  23 #include "boost/type_traits/is_pod.hpp"
  24 #include "boost/type_traits/is_stateless.hpp"
  25
  26 namespace boost {
  27
  28 struct blank
  29 {
  30 };
  31
  32 // type traits specializations
  33 //
  34
  35 template <>
  36 struct is_pod< blank >
  37     : mpl::true_
  38 {
  39 };
  40
  41 template <>
  42 struct is_empty< blank >
  43     : mpl::true_
  44 {
  45 };
  46
  47 template <>
  48 struct is_stateless< blank >
  49     : mpl::true_
  50 {
  51 };
  52
  53 // relational operators
  54 //
  55
  56 inline bool operator==(const blank&, const blank&)
  57 {
  58     return true;
  59 }
  60
  61 inline bool operator<=(const blank&, const blank&)
  62 {
  63     return true;
  64 }
  65
  66 inline bool operator>=(const blank&, const blank&)
  67 {
  68     return true;
  69 }
  70
  71 inline bool operator!=(const blank&, const blank&)
  72 {
  73     return false;
  74 }
  75
  76 inline bool operator<(const blank&, const blank&)
  77 {
  78     return false;
  79 }
  80
  81 inline bool operator>(const blank&, const blank&)
  82 {
  83     return false;
  84 }
  85
  86 // streaming support
  87 //
  88 BOOST_TEMPLATED_STREAM_TEMPLATE(E,T)
  89 inline BOOST_TEMPLATED_STREAM(ostream, E,T)& operator<<(
  90       BOOST_TEMPLATED_STREAM(ostream, E,T)& out
  91     , const blank&
  92     )
  93 {
  94     // (output nothing)
  95     return out;
  96 }
  97
  98 } // namespace boost
  99
 100 #endif // BOOST_BLANK_HPP