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<title>Memory Allocation</title>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_parser.tutorial.memory_allocation"></a><a class="link" href="memory_allocation.html" title="Memory Allocation">Memory Allocation</a>
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</h3></div></div></div>
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<p>
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Boost.Parser seldom allocates memory. The exceptions to this are:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<code class="computeroutput"><a class="link" href="../../boost/parser/symbols.html" title="Struct template symbols">symbols</a></code>
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allocates memory for the symbol/attribute pairs it contains. If symbols
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are added during the parse, allocations must also occur then. The data
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structure used by <code class="computeroutput"><a class="link" href="../../boost/parser/symbols.html" title="Struct template symbols">symbols</a></code> is also a trie,
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which is a node-based tree. So, lots of allocations are likely if you
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use <code class="computeroutput"><a class="link" href="../../boost/parser/symbols.html" title="Struct template symbols">symbols</a></code>.
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</li>
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<li class="listitem">
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The error handlers that can take a file name allocate memory for the
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file name, if one is provided.
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</li>
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<li class="listitem">
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If trace is turned on by passing <code class="computeroutput"><a class="link" href="../../boost/parser/trace.html" title="Type trace">boost::parser::trace</a>::on</code> to a top-level
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parsing function, the names of parsers are allocated.
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</li>
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<li class="listitem">
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When a failed expectation is encountered (using <code class="computeroutput">operator></code>),
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the name of the failed parser is placed into a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>,
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which will usually cause an allocation.
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</li>
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<li class="listitem">
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<code class="computeroutput"><a class="link" href="../../boost/parser/string.html" title="Function template string">string()</a></code>'s attribute is a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>, the use of which implies allocation.
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You can avoid this allocation by explicitly using a different string
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type for the attribute that does not allocate.
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</li>
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<li class="listitem">
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The attribute for <code class="computeroutput"><a class="link" href="../../boost/parser/repeat_id13.html" title="Function template repeat">repeat</a>(p)</code> in
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all its forms, including <code class="computeroutput">operator*</code>, <code class="computeroutput">operator+</code>,
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and <code class="computeroutput">operator%</code>, is <code class="computeroutput">std::vector<<span class="emphasis"><em><code class="literal">ATTR</code></em></span>(p)></code>,
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the use of which implies allocation. You can avoid this allocation by
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explicitly using a different sequence container for the attribute that
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does not allocate. <code class="computeroutput">boost::container::static_vector</code> or C++26's
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<code class="computeroutput">std::inplace_vector</code> may be useful as such replacements.
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</li>
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</ul></div>
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<p>
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With the exception of allocating the name of the parser that was expected
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in a failed expectation situation, Boost.Parser does not does not allocate
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unless you tell it to, by using <code class="computeroutput"><a class="link" href="../../boost/parser/symbols.html" title="Struct template symbols">symbols</a></code>, using a particular
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error_handler, turning on trace, or parsing into attributes that allocate.
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</p>
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</div>
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<div class="copyright-footer">Copyright © 2020 T. Zachary Laine<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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