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1[#optional]
2[section optional]
3
4[h1 Synopsis]
5
6  template <class P, class Default = /* unspecified */>
7  struct optional;
8
9This 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
19It tries parsing the input with `P`. When `P` succeeds, the result of parsing is
20the result of `P`. Otherwise no characters are consumed and the result of
21parsing is `Default`.
22
23[h1 Header]
24
25  #include <boost/metaparse/optional.hpp>
26
27[h1 Expression semantics]
28
29For any `p` [link parser parser] and `d`
30[link metaprogramming_value template metaprogramming value]
31
32  optional<p, d>
33
34is 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
107