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

   1 [#is_error]
   2 [section is_error]
   3
   4 [h1 Synopsis]
   5
   6   template <class C>
   7   struct is_error;
   8
   9
  10 This is a [link lazy_metafunction lazy template metafunction] that supports
  11 [link currying currying].
  12
  13 [table Arguments
  14   [[Name] [Type]]
  15   [[`C`]  [[link accept accept] or [link reject reject] value]]
  16 ]
  17
  18 [h1 Description]
  19
  20 Determines if `C` is a parsing error or not. Returns a [link boxed_value boxed]
  21 boolean value.
  22
  23 [h1 Header]
  24
  25   #include <boost/metaparse/is_error.hpp>
  26
  27 [h1 Expression semantics]
  28
  29 For any `e` parsing error `is_error<c>::type` is a wrapped compile-time `true`
  30 value, for any other `c` class `is_error<c>::type` is a wrapped compile-time
  31 `false` value.
  32
  33 [h1 Example]
  34
  35   #include <boost/metaparse/is_error.hpp>
  36   #include <boost/metaparse/accept.hpp>
  37   #include <boost/metaparse/reject.hpp>
  38   #include <boost/metaparse/string.hpp>
  39   #include <boost/metaparse/start.hpp>
  40   #include <boost/metaparse/define_error.hpp>
  41
  42   #include <type_traits>
  43
  44   using namespace boost::metaparse;
  45
  46   BOOST_METAPARSE_DEFINE_ERROR(sample_error, "Sample error message");
  47
  48   struct returns_reject
  49   {
  50     typedef reject<sample_error, start> type;
  51   };
  52
  53   static_assert(
  54     !is_error<
  55       accept<
  56         std::integral_constant<int, 13>,
  57         BOOST_METAPARSE_STRING("foo"),
  58         start
  59       >
  60     >::type::value,
  61     "an accept should not be an error"
  62   );
  63
  64   static_assert(
  65     is_error<reject<sample_error, start>>::type::value,
  66     "an reject should be an error"
  67   );
  68
  69   static_assert(
  70     is_error<>::type::apply<reject<sample_error, start>>::type::value,
  71     "it should support currying"
  72   );
  73
  74   static_assert(
  75     is_error<returns_reject>::type::value,
  76     "it should support lazy evaluation"
  77   );
  78
  79 [endsect]
  80