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

   1 [#nth_of]
   2 [section nth_of]
   3
   4 [h1 Synopsis]
   5
   6   template <class N, class P0, /* ... */, class Pk>
   7   struct nth_of;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name]       [Type]]
  13   [[`N`]        [[link boxed_value boxed] integer value in the range `[0..k]`]]
  14   [[`P0`..`Pk`] [[link parser parser]s]]
  15 ]
  16
  17 [h1 Description]
  18
  19 `nth_of` applies the `P0` .. `Pk` parsers in sequence. It accepts an input when
  20 all of these parsers accept it. The result of parsing is the result of the `N`.
  21 parser.
  22
  23 The maximum number of parsers `nth_of` accepts can be specified with the
  24 `BOOST_METAPARSE_LIMIT_SEQUENCE_SIZE` macro. Its default value is `5`.
  25
  26 [h1 Header]
  27
  28   #include <boost/metaparse/nth_of.hpp>
  29
  30 [h1 Expression semantics]
  31
  32 For any `p0`, ..., `pn` parsers and `k` boxed integer value the following are
  33 equivalent
  34
  35   nth_of<k, p0, ..., pn>
  36
  37   nth_of_c<k::type::value, p0, ..., pn>
  38
  39 [h1 Example]
  40
  41   #include <boost/metaparse/nth_of.hpp>
  42   #include <boost/metaparse/int_.hpp>
  43   #include <boost/metaparse/lit_c.hpp>
  44   #include <boost/metaparse/token.hpp>
  45   #include <boost/metaparse/start.hpp>
  46   #include <boost/metaparse/string.hpp>
  47   #include <boost/metaparse/is_error.hpp>
  48   #include <boost/metaparse/get_result.hpp>
  49
  50   #include <type_traits>
  51
  52   using namespace boost::metaparse;
  53
  54   using int_token = token<int_>;
  55   using left_paren_token = token<lit_c<'('>>;
  56   using right_paren_token = token<lit_c<')'>>;
  57
  58   using int_in_parens =
  59     nth_of<
  60       std::integral_constant<int, 1>,
  61       left_paren_token, int_token, right_paren_token
  62     >;
  63
  64   static_assert(
  65     get_result<
  66       int_in_parens::apply<BOOST_METAPARSE_STRING("(13)"), start>
  67     >::type::value == 13,
  68     "it should return the result of the second parser"
  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 are no parens"
  76   );
  77
  78   static_assert(
  79     is_error<
  80       int_in_parens::apply<BOOST_METAPARSE_STRING("(13"), start>
  81     >::type::value,
  82     "it should reject the input when there is no closing paren"
  83   );
  84
  85 [endsect]
  86