boost/variant/static_visitor.hpp

   1 //-----------------------------------------------------------------------------
   2 // boost variant/static_visitor.hpp header file
   3 // See http://www.boost.org for updates, documentation, and revision history.
   4 //-----------------------------------------------------------------------------
   5 //
   6 // Copyright (c) 2002-2003
   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_VARIANT_STATIC_VISITOR_HPP
  14 #define BOOST_VARIANT_STATIC_VISITOR_HPP
  15
  16 #include "boost/config.hpp"
  17 #include "boost/detail/workaround.hpp"
  18
  19 #include "boost/mpl/if.hpp"
  20 #include "boost/type_traits/is_base_and_derived.hpp"
  21
  22 #if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
  23 #   include "boost/type_traits/is_same.hpp"
  24 #endif
  25
  26 // should be the last #include
  27 #include "boost/type_traits/detail/bool_trait_def.hpp"
  28
  29 namespace boost {
  30
  31 //////////////////////////////////////////////////////////////////////////
  32 // class template static_visitor
  33 //
  34 // An empty base class that typedefs the return type of a deriving static
  35 // visitor. The class is analogous to std::unary_function in this role.
  36 //
  37
  38 namespace detail {
  39
  40     struct is_static_visitor_tag { };
  41
  42     typedef void static_visitor_default_return;
  43
  44 } // namespace detail
  45
  46 template <typename R = ::boost::detail::static_visitor_default_return>
  47 class static_visitor
  48     : public detail::is_static_visitor_tag
  49 {
  50 public: // typedefs
  51
  52     typedef R result_type;
  53
  54 protected: // for use as base class only
  55
  56     static_visitor() { }
  57     ~static_visitor() { }
  58
  59 };
  60
  61 //////////////////////////////////////////////////////////////////////////
  62 // metafunction is_static_visitor
  63 //
  64 // Value metafunction indicates whether the specified type derives from
  65 // static_visitor<...>.
  66 //
  67 // NOTE #1: This metafunction does NOT check whether the specified type
  68 //  fulfills the requirements of the StaticVisitor concept.
  69 //
  70 // NOTE #2: This template never needs to be specialized!
  71 //
  72
  73 namespace detail {
  74
  75 template <typename T>
  76 struct is_static_visitor_impl
  77 {
  78     BOOST_STATIC_CONSTANT(bool, value =
  79         (::boost::is_base_and_derived<
  80             detail::is_static_visitor_tag,
  81             T
  82         >::value));
  83 };
  84
  85 } // namespace detail
  86
  87 BOOST_TT_AUX_BOOL_TRAIT_DEF1(
  88       is_static_visitor
  89     , T
  90     , (::boost::detail::is_static_visitor_impl<T>::value)
  91     )
  92
  93 } // namespace boost
  94
  95 #include "boost/type_traits/detail/bool_trait_undef.hpp"
  96
  97 #endif // BOOST_VARIANT_STATIC_VISITOR_HPP