libs/spirit/doc/qi/complex.qbk
[/==============================================================================
Copyright (C) 2001-2011 Joel de Guzman
Copyright (C) 2001-2011 Hartmut Kaiser
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]
[section Complex - Our first complex parser]
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:
# Alternates: e.g. `a | b`. Try `a` first. If it succeeds, good. If not, try the
next alternative, `b`.
# Optionals: e.g. -p. Match the parser p zero or one time.
The complex parser presented above reads as:
* One or two real numbers in parentheses, separated by comma (the second number is optional)
* *OR* a single real number.
This parser can parse complex numbers of the form:
(123.45, 987.65)
(123.45)
123.45
[import ../../example/qi/complex_number.cpp]
Here goes, this time with actions:
[tutorial_complex_number]
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. `_1`, `_2`, etc.). Please be aware
that we are not using the same placeholders supplied by Phoenix. Take
note that we are pulling in the placeholders from namespace
`boost::spirit::qi`. These placeholders are specifically tailored for
Spirit.]
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.
[endsect]
