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Well, not really a complex parser, but a parser that parses complex numbers.
Here's a simple parser expression for complex numbers:
'(' >> double_ >> -(',' >> double_) >> ')' | double_
What's new? Well, we have:
a |
b
. Try a
first. If it succeeds, good. If not, try the next alternative, b
.
The complex parser presented above reads as:
This parser can parse complex numbers of the form:
(123.45, 987.65) (123.45) 123.45
Here goes, this time with actions:
namespace client { template <typename Iterator> bool parse_complex(Iterator first, Iterator last, std::complex<double>& c) { using boost::spirit::x3::double_; using boost::spirit::x3::_attr; using boost::spirit::x3::phrase_parse; using boost::spirit::x3::ascii::space; double rN = 0.0; double iN = 0.0; auto fr = [&](auto& ctx){ rN = _attr(ctx); }; auto fi = [&](auto& ctx){ iN = _attr(ctx); }; bool r = phrase_parse(first, last, // Begin grammar ( '(' >> double_[fr] >> -(',' >> double_[fi]) >> ')' | double_[fr] ), // End grammar space); if (!r || first != last) // fail if we did not get a full match return false; c = std::complex<double>(rN, iN); return r; } }
The full cpp file for this example can be found here: complex_number.cpp
The double_
parser attaches
this action:
[&](auto& ctx){ n = _attr(ctx); }
This assigns the parsed result (actually, the attribute of double_
) to n.