1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
|
[/==============================================================================
Copyright (C) 2001-2015 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.
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
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:
[@../../../example/x3/complex_number.cpp 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.
[endsect]
|
