81 lines
2.4 KiB
Plaintext
81 lines
2.4 KiB
Plaintext
[/==============================================================================
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Copyright (C) 2001-2015 Joel de Guzman
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Copyright (C) 2001-2011 Hartmut Kaiser
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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===============================================================================/]
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[section Complex - Our first complex parser]
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Well, not really a complex parser, but a parser that parses complex numbers.
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Here's a simple parser expression for complex numbers:
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'(' >> double_ >> -(',' >> double_) >> ')'
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| double_
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What's new? Well, we have:
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# Alternates: e.g. `a | b`. Try `a` first. If it succeeds, good. If not, try the
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next alternative, `b`.
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# Optionals: e.g. -p. Match the parser p zero or one time.
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The complex parser presented above reads as:
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* One or two real numbers in parentheses, separated by comma (the second number is optional)
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* *OR* a single real number.
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This parser can parse complex numbers of the form:
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(123.45, 987.65)
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(123.45)
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123.45
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Here goes, this time with actions:
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namespace client
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{
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template <typename Iterator>
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bool parse_complex(Iterator first, Iterator last, std::complex<double>& c)
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{
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using boost::spirit::x3::double_;
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using boost::spirit::x3::_attr;
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using boost::spirit::x3::phrase_parse;
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using boost::spirit::x3::ascii::space;
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double rN = 0.0;
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double iN = 0.0;
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auto fr = [&](auto& ctx){ rN = _attr(ctx); };
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auto fi = [&](auto& ctx){ iN = _attr(ctx); };
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bool r = phrase_parse(first, last,
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// Begin grammar
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(
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'(' >> double_[fr]
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>> -(',' >> double_[fi]) >> ')'
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| double_[fr]
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),
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// End grammar
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space);
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if (!r || first != last) // fail if we did not get a full match
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return false;
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c = std::complex<double>(rN, iN);
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return r;
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}
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}
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The full cpp file for this example can be found here:
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[@../../../example/x3/complex_number.cpp complex_number.cpp]
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The `double_` parser attaches this action:
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[&](auto& ctx){ n = _attr(ctx); }
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This assigns the parsed result (actually, the attribute of `double_`) to n.
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[endsect]
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