ceph/src/boost/libs/spirit/doc/x3/tutorial/complex.qbk

   1 [/==============================================================================
   2     Copyright (C) 2001-2015 Joel de Guzman
   3     Copyright (C) 2001-2011 Hartmut Kaiser
   4
   5     Distributed under the Boost Software License, Version 1.0. (See accompanying
   6     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   7 ===============================================================================/]
   8
   9 [section Complex - Our first complex parser]
  10
  11 Well, not really a complex parser, but a parser that parses complex numbers.
  12
  13 Here's a simple parser expression for complex numbers:
  14
  15         '(' >> double_ >> -(',' >> double_) >> ')'
  16     |   double_
  17
  18 What's new? Well, we have:
  19
  20 # Alternates: e.g. `a | b`. Try `a` first. If it succeeds, good. If not, try the
  21   next alternative, `b`.
  22 # Optionals: e.g. -p. Match the parser p zero or one time.
  23
  24 The complex parser presented above reads as:
  25
  26 * One or two real numbers in parentheses, separated by comma (the second number is optional)
  27 * *OR* a single real number.
  28
  29 This parser can parse complex numbers of the form:
  30
  31     (123.45, 987.65)
  32     (123.45)
  33     123.45
  34
  35 Here goes, this time with actions:
  36
  37     namespace client
  38     {
  39         template <typename Iterator>
  40         bool parse_complex(Iterator first, Iterator last, std::complex<double>& c)
  41         {
  42             using boost::spirit::x3::double_;
  43             using boost::spirit::x3::_attr;
  44             using boost::spirit::x3::phrase_parse;
  45             using boost::spirit::x3::ascii::space;
  46
  47             double rN = 0.0;
  48             double iN = 0.0;
  49             auto fr = [&](auto& ctx){ rN = _attr(ctx); };
  50             auto fi = [&](auto& ctx){ iN = _attr(ctx); };
  51
  52             bool r = phrase_parse(first, last,
  53
  54                 //  Begin grammar
  55                 (
  56                         '(' >> double_[fr]
  57                             >> -(',' >> double_[fi]) >> ')'
  58                     |   double_[fr]
  59                 ),
  60                 //  End grammar
  61
  62                 space);
  63
  64             if (!r || first != last) // fail if we did not get a full match
  65                 return false;
  66             c = std::complex<double>(rN, iN);
  67             return r;
  68         }
  69     }
  70
  71 The full cpp file for this example can be found here: [@../../../example/x3/complex_number.cpp]
  72
  73 The `double_` parser attaches this action:
  74
  75     [&](auto& ctx){ n = _attr(ctx); }
  76
  77 This assigns the parsed result (actually, the attribute of `double_`) to n.
  78
  79 [endsect]