60 lines
2.0 KiB
Plaintext
60 lines
2.0 KiB
Plaintext
[/==============================================================================
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Copyright (C) 2001-2011 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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This time, we're using __phoenix__ to do the semantic actions.
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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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[import ../../example/qi/complex_number.cpp]
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Here goes, this time with actions:
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[tutorial_complex_number]
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The full cpp file for this example can be found here: [@../../example/qi/complex_number.cpp]
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[note Those with experience using __phoenix__ might be confused with the
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placeholders that we are using (i.e. `_1`, `_2`, etc.). Please be aware
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that we are not using the same placeholders supplied by Phoenix. Take
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note that we are pulling in the placeholders from namespace
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`boost::spirit::qi`. These placeholders are specifically tailored for
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Spirit.]
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The `double_` parser attaches this action:
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ref(n) = _1
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This assigns the parsed result (actually, the attribute of `double_`) to n.
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`ref(n)` tells Phoenix that `n` is a mutable reference. `_1` is a Phoenix
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placeholder for the parsed result attribute.
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[endsect]
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