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

   1 [#if_]
   2 [section if_]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class T, class F>
   7   struct if_;
   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   [[`F`]  [[link metaprogramming_value template metaprogramming value]]]
  16 ]
  17
  18 [h1 Description]
  19
  20 `if_` always accepts the input string. The result of parsing is `T`, when `P`
  21 accepts the input and `F` otherwise.
  22
  23 [h1 Header]
  24
  25   #include <boost/metaparse/if_.hpp>
  26
  27 [h1 Expression semantics]
  28
  29 For any `p` parser, `t` and `f` classes the following are equivalent:
  30
  31   if_<p, t, f>
  32
  33   one_of<last_of<p, return_<t>>, return_<f>>
  34
  35 [h1 Example]
  36
  37   #include <boost/metaparse/if_.hpp>
  38   #include <boost/metaparse/int_.hpp>
  39   #include <boost/metaparse/string.hpp>
  40   #include <boost/metaparse/start.hpp>
  41   #include <boost/metaparse/get_result.hpp>
  42
  43   #include <type_traits>
  44
  45   using namespace boost::metaparse;
  46
  47   using int11 = std::integral_constant<int, 11>;
  48   using int13 = std::integral_constant<int, 13>;
  49
  50   static_assert(
  51     get_result<
  52       if_<int_, int11, int13>::apply<BOOST_METAPARSE_STRING("1234"), start>
  53     >::type::value == 11,
  54     "When the int_ parser succeeds, the result of parsing should be 11"
  55   );
  56
  57   static_assert(
  58     get_result<
  59       if_<int_, int11, int13>::apply<BOOST_METAPARSE_STRING("foo"), start>
  60     >::type::value == 13,
  61     "When the int_ parser fails, the result of parsing should be 13"
  62   );
  63
  64 [endsect]
  65