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

   1 [#middle_of]
   2 [section middle_of]
   3
   4 [h1 Synopsis]
   5
   6   template <class P1, class P2, class P3>
   7   struct middle_of;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name] [Type]]
  13   [[`P1`] [[link parser parser]]]
  14   [[`P2`] [[link parser parser]]]
  15   [[`P3`] [[link parser parser]]]
  16 ]
  17
  18 [h1 Description]
  19
  20 `middle_of` applies `P1`, `P2` and `P3` in sequence. It accepts an input when
  21 all of these three parsers accept it. The result of parsing is the result of
  22 `P2`.
  23
  24 [h1 Header]
  25
  26   #include <boost/metaparse/middle_of.hpp>
  27
  28 [h1 Expression semantics]
  29
  30 For any `p1`, `p2` and `p3` parsers
  31
  32   middle_of<p1, p2, p3>
  33
  34 is equivalent to
  35
  36   nth_of<1, p1, p2, p3>
  37
  38 [h1 Example]
  39
  40   #include <boost/metaparse/middle_of.hpp>
  41   #include <boost/metaparse/int_.hpp>
  42   #include <boost/metaparse/lit_c.hpp>
  43   #include <boost/metaparse/token.hpp>
  44   #include <boost/metaparse/start.hpp>
  45   #include <boost/metaparse/string.hpp>
  46   #include <boost/metaparse/is_error.hpp>
  47   #include <boost/metaparse/get_result.hpp>
  48
  49   using namespace boost::metaparse;
  50
  51   using int_token = token<int_>;
  52   using left_paren_token = token<lit_c<'('>>;
  53   using right_paren_token = token<lit_c<')'>>;
  54
  55   using int_in_parens = middle_of<left_paren_token, int_token, right_paren_token>;
  56
  57   static_assert(
  58     get_result<
  59       int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start>
  60     >::type::value == 13,
  61     "it should return the result of the middle parser"
  62   );
  63
  64   static_assert(
  65     is_error<
  66       int_in_parens::apply<BOOST_METAPARSE_STRING("13"), start>
  67     >::type::value,
  68     "it should reject the input when there are no parens"
  69   );
  70
  71   static_assert(
  72     is_error<
  73       int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start>
  74     >::type::value,
  75     "it should reject the input when there is no closing paren"
  76   );
  77
  78 [endsect]
  79