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

   1 [#return_]
   2 [section return_]
   3
   4 [h1 Synopsis]
   5
   6   template <class C>
   7   struct return_;
   8
   9 This is a [link parser parser].
  10
  11 [table Arguments
  12   [[Name] [Type]]
  13   [[`C`]  [[link metaprogramming_value template metaprogramming value]]]
  14 ]
  15
  16 [h1 Description]
  17
  18 `return_` accepts every input. The result of parsing is `C`, the remaining
  19 string is the input string.
  20
  21 [h1 Header]
  22
  23   #include <boost/metaparse/return_.hpp>
  24
  25 [h1 Expression semantics]
  26
  27 For any `c` class, `s` compile-time string and `pos` source position the
  28 following are equivalent
  29
  30   get_result<return_<c>::apply<s, pos>>::type
  31
  32   c
  33
  34   get_remaining<return_<c>::apply<s, pos>>::type
  35
  36   s
  37
  38   get_position<return_<c>::apply<s, pos>>::type
  39
  40   pos
  41
  42 [h1 Example]
  43
  44   #include <boost/metaparse/return_.hpp>
  45   #include <boost/metaparse/int_.hpp>
  46   #include <boost/metaparse/one_of.hpp>
  47   #include <boost/metaparse/start.hpp>
  48   #include <boost/metaparse/string.hpp>
  49   #include <boost/metaparse/get_result.hpp>
  50
  51   #include <type_traits>
  52
  53   using namespace boost::metaparse;
  54
  55   using default_value = std::integral_constant<int, 13>;
  56
  57   using optional_number = one_of<int_, return_<default_value>>;
  58
  59   static_assert(
  60     get_result<
  61       optional_number::apply<BOOST_METAPARSE_STRING("11"), start>
  62     >::type::value == 11,
  63     "when a number is provided, it is the result of parsing"
  64   );
  65
  66   static_assert(
  67     get_result<
  68       optional_number::apply<BOOST_METAPARSE_STRING(""), start>
  69     >::type::value == 13,
  70     "when no number is provided, the default value is the result of parsing"
  71   );
  72
  73 [endsect]
  74