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

   1 [#always]
   2 [section always]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class T>
   7   struct always;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name] [Type]]
  13   [[`P`]  [[link parser parser]]]
  14   [[`T`]  [[link metaprogramming_value template metaprogramming value]]]
  15 ]
  16
  17 [h1 Description]
  18
  19 It accepts an input if and only if `P` accepts it, but the result of parsing
  20 will be `T` instead of the result `P` returned.
  21
  22 [h1 Header]
  23
  24   #include <boost/metaparse/always.hpp>
  25
  26 [h1 Expression semantics]
  27
  28 For any `p` parser and `t` class the following are equivalent:
  29
  30   always<p, t>
  31
  32   last_of<p, return_<t>>
  33
  34 [h1 Example]
  35
  36   #include <boost/metaparse/always.hpp>
  37   #include <boost/metaparse/lit_c.hpp>
  38   #include <boost/metaparse/start.hpp>
  39   #include <boost/metaparse/string.hpp>
  40   #include <boost/metaparse/is_error.hpp>
  41   #include <boost/metaparse/get_result.hpp>
  42
  43   #include <type_traits>
  44
  45   using namespace boost::metaparse;
  46
  47   using always13 = always<lit_c<'x'>, std::integral_constant<int, 13>>;
  48
  49   static_assert(
  50     !is_error<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type::value,
  51     "always13 should accept x"
  52   );
  53
  54   static_assert(
  55     std::is_same<
  56       get_result<always13::apply<BOOST_METAPARSE_STRING("x"), start>>::type,
  57       std::integral_constant<int, 13>
  58     >::type::value,
  59     "the result of parsing should be the integral_constant type"
  60   );
  61
  62   static_assert(
  63     is_error<always13::apply<BOOST_METAPARSE_STRING("y"), start>>::type::value,
  64     "always13 should reject characters other than x"
  65   );
  66
  67 [endsect]
  68