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

   1 [#optional]
   2 [section optional]
   3
   4 [h1 Synopsis]
   5
   6   template <class P, class Default = /* unspecified */>
   7   struct optional;
   8
   9 This is a [link parser_combinator parser combinator].
  10
  11 [table Arguments
  12   [[Name]   [Type]]
  13   [[`P`]    [[link parser parser]]]
  14   [[`Default`] [[link metaprogramming_value template metaprogramming value]]]
  15 ]
  16
  17 [h1 Description]
  18
  19 It tries parsing the input with `P`. When `P` succeeds, the result of parsing is
  20 the result of `P`. Otherwise no characters are consumed and the result of
  21 parsing is `Default`.
  22
  23 [h1 Header]
  24
  25   #include <boost/metaparse/optional.hpp>
  26
  27 [h1 Expression semantics]
  28
  29 For any `p` [link parser parser] and `d`
  30 [link metaprogramming_value template metaprogramming value]
  31
  32   optional<p, d>
  33
  34 is equivalent to
  35
  36   one_of<p, return_<d>>
  37
  38 [h1 Example]
  39
  40   #include <boost/metaparse/optional.hpp>
  41   #include <boost/metaparse/start.hpp>
  42   #include <boost/metaparse/int_.hpp>
  43   #include <boost/metaparse/middle_of.hpp>
  44   #include <boost/metaparse/sequence.hpp>
  45   #include <boost/metaparse/lit_c.hpp>
  46   #include <boost/metaparse/string.hpp>
  47   #include <boost/metaparse/get_result.hpp>
  48
  49   #include <boost/mpl/int.hpp>
  50   #include <boost/mpl/equal.hpp>
  51   #include <boost/mpl/equal_to.hpp>
  52   #include <boost/mpl/vector_c.hpp>
  53
  54   using namespace boost::metaparse;
  55
  56   using complex_number =
  57     sequence<
  58       // Real
  59       int_,
  60
  61       // Imaginary
  62       optional<
  63         middle_of<lit_c<'+'>, int_, lit_c<'i'>>,
  64         boost::mpl::int_<0>
  65       >
  66     >
  67   ;
  68
  69   static_assert(
  70     boost::mpl::equal<
  71       boost::mpl::vector_c<int, 1, 0>,
  72       get_result<
  73         complex_number::apply<BOOST_METAPARSE_STRING("1"), start>
  74       >::type,
  75
  76       boost::mpl::equal_to<boost::mpl::_, boost::mpl::_>
  77     >::type::value,
  78     "No imaginary"
  79   );
  80
  81   static_assert(
  82     boost::mpl::equal<
  83       boost::mpl::vector_c<int, 1, 0>,
  84       get_result<
  85         complex_number::apply<BOOST_METAPARSE_STRING("1+0i"), start>
  86       >::type,
  87
  88       boost::mpl::equal_to<boost::mpl::_, boost::mpl::_>
  89     >::type::value,
  90     "0 as imaginary"
  91   );
  92
  93   static_assert(
  94     boost::mpl::equal<
  95       boost::mpl::vector_c<int, 0, 1>,
  96       get_result<
  97         complex_number::apply<BOOST_METAPARSE_STRING("0+1i"), start>
  98       >::type,
  99
 100       boost::mpl::equal_to<boost::mpl::_, boost::mpl::_>
 101     >::type::value,
 102     "Non-null imaginary"
 103   );
 104
 105 [endsect]
 106