ceph/src/boost/libs/metaparse/doc/unless_error.qbk

   1 [#unless_error]
   2 [section unless_error]
   3
   4 [h1 Synopsis]
   5
   6   template <class T, class NotErrorCase>
   7   struct unless_error;
   8
   9 This is a [link lazy_metafunction lazy template metafunction].
  10
  11 [table Arguments
  12   [[Name]           [Type]]
  13   [[`T`]            [[link accept accept] or [link reject reject] value]]
  14   [[`NotErrorCase`] [[link metaprogramming_value template metaprogramming value]]]
  15 ]
  16
  17 [h1 Description]
  18
  19 Checks if `T` is a parsing error or not. When it is, the result is `T`. When it
  20 is not, the result is `NotErrorCase`.
  21
  22 [h1 Header]
  23
  24   #include <boost/metaparse/unless_error.hpp>
  25
  26 [h1 Expression semantics]
  27
  28 For any `t` and `c` classes the following are equivalent:
  29
  30   unless_error<t, c>
  31
  32   boost::mpl::if_<is_error<t::type>::type, t, c>
  33
  34 [h1 Example]
  35
  36   #include <boost/metaparse/unless_error.hpp>
  37   #include <boost/metaparse/accept.hpp>
  38   #include <boost/metaparse/reject.hpp>
  39   #include <boost/metaparse/start.hpp>
  40   #include <boost/metaparse/string.hpp>
  41   #include <boost/metaparse/define_error.hpp>
  42
  43   #include <type_traits>
  44
  45   using namespace boost::metaparse;
  46
  47   BOOST_METAPARSE_DEFINE_ERROR(sample_error, "Sample error message");
  48
  49   using accept1 =
  50     accept<std::integral_constant<int, 11>, BOOST_METAPARSE_STRING("foo"), start>;
  51
  52   using accept2 =
  53     accept<std::integral_constant<int, 13>, BOOST_METAPARSE_STRING("bar"), start>;
  54
  55   using reject1 = reject<sample_error, start>;
  56
  57   struct returns_accept1 { using type = accept1; };
  58   struct returns_accept2 { using type = accept2; };
  59
  60   static_assert(
  61     std::is_same<accept2, unless_error<accept1, accept2>::type>::type::value,
  62     "it returns the second argument when the first argument is an accept"
  63   );
  64
  65   static_assert(
  66     std::is_same<reject1, unless_error<reject1, accept2>::type>::type::value,
  67     "it returns the first argument when that is a reject"
  68   );
  69
  70   static_assert(
  71     std::is_same<
  72       accept2,
  73       unless_error<returns_accept1, returns_accept2>::type
  74     >::type::value,
  75     "it supports lazy evaluation"
  76   );
  77
  78 [endsect]
  79