...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Well, not really a complex parser, but a parser that parses complex numbers. This time, we're using Phoenix to do the semantic actions.
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::qi::double_; using boost::spirit::qi::_1; using boost::spirit::qi::phrase_parse; using boost::spirit::ascii::space; using boost::phoenix::ref; double rN = 0.0; double iN = 0.0; bool r = phrase_parse(first, last, // Begin grammar ( '(' >> double_[ref(rN) = _1] >> -(',' >> double_[ref(iN) = _1]) >> ')' | double_[ref(rN) = _1] ), // 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: ../../example/qi/complex_number.cpp
Note | |
---|---|
Those with experience using Phoenix
might be confused with the placeholders that we are using (i.e. |
The double_
parser attaches
this action:
ref(n) = _1
This assigns the parsed result (actually, the attribute of double_
) to n. ref(n)
tells Phoenix that n
is
a mutable reference. _1
is a Phoenix placeholder for the parsed result attribute.