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
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Alternative Parser (a | b)</title>
<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.76.1">
<link rel="home" href="../../../../index.html" title="Spirit 2.5.2">
<link rel="up" href="../operator.html" title="Parser Operators">
<link rel="prev" href="../operator.html" title="Parser Operators">
<link rel="next" href="and_predicate.html" title="And-Predicate Parser (&a)">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../operator.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../operator.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="and_predicate.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.operator.alternative"></a><a class="link" href="alternative.html" title="Alternative Parser (a | b)">Alternative
Parser (<code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span>
<span class="identifier">b</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h0"></a>
<span><a name="spirit.qi.reference.operator.alternative.description"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.description">Description</a>
</h6>
<p>
The alternative operator, <code class="computeroutput"><span class="identifier">a</span>
<span class="special">|</span> <span class="identifier">b</span></code>,
matches one of two or more operands (<code class="computeroutput"><span class="identifier">a</span></code>,
<code class="computeroutput"><span class="identifier">b</span></code>, ... etc.):
</p>
<pre class="programlisting"><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="special">...</span>
</pre>
<p>
Alternative operands are tried one by one on a first-match-wins basis
starting from the leftmost operand. After a successfully matched alternative
is found, the parser concludes its search, essentially short-circuiting
the search for other potentially viable candidates. This short-circuiting
implicitly gives the highest priority to the leftmost alternative.
</p>
<p>
Short-circuiting is done in the same manner as C or C++'s logical expressions;
e.g. <code class="computeroutput"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special"><</span> <span class="number">3</span> <span class="special">||</span> <span class="identifier">y</span>
<span class="special"><</span> <span class="number">2</span><span class="special">)</span></code> where, if <code class="computeroutput"><span class="identifier">x</span>
<span class="special"><</span> <span class="number">3</span></code>,
the <code class="computeroutput"><span class="identifier">y</span> <span class="special"><</span>
<span class="number">2</span></code> test is not done at all. In addition
to providing an implicit priority rule for alternatives which is necessary,
given its non-deterministic nature, short-circuiting improves the execution
time. If the order of your alternatives is logically irrelevant, strive
to put the (expected) most common choice first for maximum efficiency.
</p>
<h6>
<a name="spirit.qi.reference.operator.alternative.h1"></a>
<span><a name="spirit.qi.reference.operator.alternative.header"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/alternative.hpp></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_alternative</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
</p>
<h6>
<a name="spirit.qi.reference.operator.alternative.h2"></a>
<span><a name="spirit.qi.reference.operator.alternative.model_of"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.model_of">Model of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a>
</p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code>, <code class="computeroutput"><span class="identifier">b</span></code></span></dt>
<dd><p>
A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>
</p></dd>
</dl>
</div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h3"></a>
<span><a name="spirit.qi.reference.operator.alternative.expression_semantics"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.expression_semantics">Expression
Semantics</a>
</h6>
<p>
Semantics of an expression is defined only where it differs from, or
is not defined in <a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</span></code></a>.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Semantics
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span>
<span class="identifier">b</span></code>
</p>
</td>
<td>
<p>
Match <code class="computeroutput"><span class="identifier">a</span></code> or
<code class="computeroutput"><span class="identifier">b</span></code>.
</p>
</td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h4"></a>
<span><a name="spirit.qi.reference.operator.alternative.attributes"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.attributes">Attributes</a>
</h6>
<p>
See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound
Attribute Notation</a>.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Attribute
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">a</span> <span class="special">|</span>
<span class="identifier">b</span></code>
</p>
</td>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">variant</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">c</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="identifier">c</span><span class="special">):</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span> <span class="special">></span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">B</span><span class="special">></span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">Unused</span>
<span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span><span class="special">,</span> <span class="identifier">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">A</span></pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
Alternative parsers do not roll back changes made to the outer attribute
because of a failed alternative. If you need to enforce that only the
succeeded alternative changes the outer attribute please utilize the
directive <a class="link" href="../directive/hold.html" title="Parser Directive for Attribute Commit/Rollback (hold[])"><code class="computeroutput"><span class="identifier">hold</span></code></a><code class="computeroutput"><span class="special">[]</span></code>.
</p></td></tr>
</table></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h5"></a>
<span><a name="spirit.qi.reference.operator.alternative.complexity"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The overall complexity of the alternative parser is defined by the
sum of the complexities of its elements. The complexity of the alternative
parser itself is O(N), where N is the number of alternatives.
</p></blockquote></div>
<h6>
<a name="spirit.qi.reference.operator.alternative.h6"></a>
<span><a name="spirit.qi.reference.operator.alternative.example"></a></span><a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.example">Example</a>
</h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a>
section.
</p></td></tr>
</table></div>
<p>
Some using declarations:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">_1</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Simple usage:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span>
<span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
Extracting the attribute variant (using <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">attr</span><span class="special">;</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
This should print <code class="computeroutput"><span class="string">"Hello"</span></code>.
Note: There are better ways to extract the value from the variant. See
<a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>
visitation. This code is solely for demonstration.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(&</span><span class="identifier">attr</span><span class="special">))</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">else</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Extracting the attributes using Semantic Actions with <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>
(this should print <code class="computeroutput"><span class="number">123</span></code>):
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="special">(</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">)[</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">_1</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">]);</span>
</pre>
<p>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="../operator.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../operator.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="and_predicate.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
|
