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<html>
<head>
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<title>Plus Generator (+a)</title>
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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.karma.reference.operator.plus"></a><a class="link" href="plus.html" title="Plus Generator (+a)">Plus Generator
(<code class="computeroutput"><span class="special">+</span><span class="identifier">a</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.karma.reference.operator.plus.h0"></a>
<span><a name="spirit.karma.reference.operator.plus.description"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.description">Description</a>
</h6>
<p>
The Plus generator is used to repeat the execution of an embedded generator
one or more times. It succeeds if the embedded generator has been successfully
executed at least once.
</p>
<h6>
<a name="spirit.karma.reference.operator.plus.h1"></a>
<span><a name="spirit.karma.reference.operator.plus.header"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/karma/operator/plus.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">karma_plus</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.karma.reference.operator.plus.h2"></a>
<span><a name="spirit.karma.reference.operator.plus.model_of"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.model_of">Model
of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</span></code></a>
</p></blockquote></div>
<h6>
<a name="spirit.karma.reference.operator.plus.h3"></a>
<span><a name="spirit.karma.reference.operator.plus.expression_semantics"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.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="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</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="special">+</span><span class="identifier">a</span></code>
</p>
</td>
<td>
<p>
The generator <code class="computeroutput"><span class="identifier">a</span></code>
is executed one or more times depending on the availability
of an attribute. The execution of <code class="computeroutput"><span class="identifier">a</span></code>
stops after the attribute values passed to the plus generator
are exhausted. The plus generator succeeds as long as its embedded
generator has been successfully executed at least once (unless
the underlying output stream reports an error).
</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>
All failing iterations of the embedded generator will consume one element
from the supplied attribute. The overall <code class="computeroutput"><span class="special">+</span><span class="identifier">a</span></code> will succeed as long as at least
one invocation of the embedded generator will succeed (unless the underlying
output stream reports an error).
</p></td></tr>
</table></div>
<h6>
<a name="spirit.karma.reference.operator.plus.h4"></a>
<span><a name="spirit.karma.reference.operator.plus.attributes"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.attributes">Attributes</a>
</h6>
<p>
See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.karma.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="special">+</span><span class="identifier">a</span></code>
(unary <code class="computeroutput"><span class="special">+</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="special">+</span><span class="identifier">a</span><span class="special">:</span> <span class="identifier">vector</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">Unused</span> <span class="special">--></span> <span class="special">+</span><span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span></pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top"><p>
The table above uses <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span></code> as a placeholder only. The notation
of <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span></code>
stands for <span class="emphasis"><em>any STL container</em></span> holding elements
of type <code class="computeroutput"><span class="identifier">A</span></code>.
</p></td></tr>
</table></div>
<p>
The Plus generator will execute its embedded generator once for each
element in the provided container attribute as long as the embedded generator
succeeds. On each iteration it will pass the next consecutive element
from the container attribute to the embedded generator. Therefore the
number of iterations will not be larger than the number of elements in
the container passed as its attribute. An empty container will make the
plus generator fail.
</p>
<p>
It is important to note, that the plus generator does not perform any
buffering of the output generated by its embedded elements. That means
that any failing element generator might have already generated some
output, which is <span class="emphasis"><em>not</em></span> rolled back.
</p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../../images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top">
<p>
The simplest way to force a plus generator to behave as if it did buffering
is to wrap it into a buffering directive (see <a class="link" href="../directive/buffer.html" title="Generator Directive for Temporary Output Buffering (buffer[])"><code class="computeroutput"><span class="identifier">buffer</span></code></a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">buffer</span><span class="special">[+</span><span class="identifier">a</span><span class="special">]</span></pre>
<p>
</p>
<p>
which will <span class="emphasis"><em>not</em></span> generate any output in case of
a failing generator <code class="computeroutput"><span class="special">+</span><span class="identifier">a</span></code>. The expression:
</p>
<p>
</p>
<pre class="programlisting"><span class="special">+(</span><span class="identifier">buffer</span><span class="special">[</span><span class="identifier">a</span><span class="special">])</span></pre>
<p>
</p>
<p>
will not generate any partial output from a generator <code class="computeroutput"><span class="identifier">a</span></code> if it fails generating in the middle
of its output. The overall expression will still generate the output
as produced by all successful invocations of the generator <code class="computeroutput"><span class="identifier">a</span></code>.
</p>
</td></tr>
</table></div>
<h6>
<a name="spirit.karma.reference.operator.plus.h5"></a>
<span><a name="spirit.karma.reference.operator.plus.complexity"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The overall complexity of the plus generator is defined by the complexity
of its embedded generator multiplied by the number of executed iterations.
The complexity of the plus generator itself is O(N), where N is the
number of elements in the container passed as its attribute.
</p></blockquote></div>
<h6>
<a name="spirit.karma.reference.operator.plus.h6"></a>
<span><a name="spirit.karma.reference.operator.plus.example"></a></span><a class="link" href="plus.html#spirit.karma.reference.operator.plus.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.karma.reference.basics.examples">Basics Examples</a>
section.
</p></td></tr>
</table></div>
<p>
Some includes:
</p>
<p>
</p>
<pre class="programlisting"><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">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">support_utree</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></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">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">string</span><span class="special">></span>
</pre>
<p>
</p>
<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">karma</span><span class="special">::</span><span class="identifier">double_</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">karma</span><span class="special">::</span><span class="identifier">space</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Basic usage of a plus generator:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v1</span><span class="special">;</span>
<span class="identifier">v1</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">1.0</span><span class="special">);</span>
<span class="identifier">v1</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">2.0</span><span class="special">);</span>
<span class="identifier">v1</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="number">3.0</span><span class="special">);</span>
<span class="identifier">test_generator_attr_delim</span><span class="special">(</span><span class="string">"1.0 2.0 3.0 "</span><span class="special">,</span> <span class="special">+</span><span class="identifier">double_</span><span class="special">,</span> <span class="identifier">space</span><span class="special">,</span> <span class="identifier">v1</span><span class="special">);</span>
</pre>
<p>
</p>
<p>
A more sophisticated use case showing how to leverage the fact that plus
is failing for empty containers passed as its attribute:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v2</span><span class="special">;</span> <span class="comment">// empty container</span>
<span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"empty"</span><span class="special">,</span> <span class="special">+</span><span class="identifier">double_</span> <span class="special">|</span> <span class="string">"empty"</span><span class="special">,</span> <span class="identifier">v2</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>
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