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

   1 [#empty]
   2 [section empty]
   3
   4 [h1 Synopsis]
   5
   6   template <class Result>
   7   struct empty;
   8
   9 This is a [link parser parser].
  10
  11 [table Arguments
  12   [[Name]     [Type]]
  13   [[`Result`] [[link metaprogramming_value template metaprogramming value]]]
  14 ]
  15
  16 [h1 Description]
  17
  18 It accepts empty input only. The result of parsing is the `Result`
  19 argument.
  20
  21 [h1 Header]
  22
  23   #include <boost/metaparse/empty.hpp>
  24
  25 [h1 Expression semantics]
  26
  27 For any `c` class the following are equivalent:
  28
  29   empty<c>
  30
  31   except<one_char, c, error::end_of_input_expected>
  32
  33 [h1 Example]
  34
  35   #include <boost/metaparse/empty.hpp>
  36   #include <boost/metaparse/start.hpp>
  37   #include <boost/metaparse/string.hpp>
  38   #include <boost/metaparse/is_error.hpp>
  39   #include <boost/metaparse/get_result.hpp>
  40
  41   #include <type_traits>
  42
  43   using namespace boost::metaparse;
  44
  45   using want_empty = empty<BOOST_METAPARSE_STRING("result")>;
  46
  47   static_assert(
  48     !is_error<want_empty::apply<BOOST_METAPARSE_STRING(""), start>>::type::value,
  49     "empty accepts the empty input"
  50   );
  51
  52   static_assert(
  53     is_error<want_empty::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
  54     "empty should reject non-empty input"
  55   );
  56
  57   static_assert(
  58     std::is_same<
  59       get_result<want_empty::apply<BOOST_METAPARSE_STRING(""), start>>::type,
  60       BOOST_METAPARSE_STRING("result")
  61     >::type::value,
  62     "the result of parsing should be the given value"
  63   );
  64
  65 [endsect]
  66