boost_1_45_0/libs/spirit/doc/qi/complex.qbk

[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2010 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]