mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
239 lines
10 KiB
HTML
239 lines
10 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Dbstl memory management</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="stl.html" title="Chapter 7. Standard Template Library API" />
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<link rel="prev" href="stl_efficienct_use.html" title="Using dbstl efficiently" />
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<link rel="next" href="stl_misc.html" title="Dbstl miscellaneous notes" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Dbstl memory management</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="stl_efficienct_use.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 7. Standard Template Library API</th>
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<td width="20%" align="right"> <a accesskey="n" href="stl_misc.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="stl_memory_mgmt"></a>Dbstl memory management</h2>
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</div>
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</div>
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</div>
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<div class="toc">
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<dl>
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<dt>
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<span class="sect2">
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<a href="stl_memory_mgmt.html#idm140654539144272">Freeing memory</a>
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</span>
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</dt>
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<dt>
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<span class="sect2">
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<a href="stl_memory_mgmt.html#idm140654539118560">Type specific notes</a>
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</span>
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</dt>
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</dl>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idm140654539144272"></a>Freeing memory</h3>
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</div>
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</div>
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</div>
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<p>
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When using dbstl, make sure memory allocated in the
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heap is released after use. The rules for this are:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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dbstl will free/delete any memory allocated by
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dbstl itself.
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</p>
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</li>
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<li>
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<p>
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You are responsible for freeing/deleting any
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memory allocated by your code outside of dbstl.
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</p>
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</li>
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</ul>
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</div>
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</div>
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<div class="sect2" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h3 class="title"><a id="idm140654539118560"></a>Type specific notes</h3>
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</div>
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</div>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h4 class="title"><a id="idm140654539120080"></a>DbEnv/Db</h4>
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</div>
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</div>
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</div>
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<p>
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When you open a <code class="classname">DbEnv</code> or
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<code class="classname">Db</code> object using
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<code class="methodname">dbstl::open_env()</code> or
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<code class="methodname">dbstl::open_db()</code>, you do
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not need to delete that object. However, if you new'd
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that object and then opened it without using the
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<code class="methodname">dbstl::open_env()</code> or
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<code class="methodname">dbstl::open_db()</code> methods,
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you are responsible for deleting the object.
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</p>
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<p>
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Note that you must <code class="literal">new</code> the
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<code class="classname">Db</code> or
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<code class="classname">DbEnv</code> object, which
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allocates it on the heap. You can not allocate it on
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the stack. If you do, the order of destruction is
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uncontrollable, which makes dbstl unable to work
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properly.
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</p>
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<p>
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You can call <code class="function">dbstl_exit()</code>
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before the process exits, to release any memory
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allocated by dbstl that has to live during the entire
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process lifetime. Releasing the memory explicitly will
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not make much difference, because the process is about
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to exit and so all memory allocated on the heap is
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going to be returned to the operating system anyway.
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The only real difference is that your memory leak
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checker will not report false memory leaks.
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</p>
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<p>
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<code class="function">dbstl_exit()</code> releases any memory
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allocated by dbstl on the heap. It also performs other
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required shutdown operations, such as closing any
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databases and environments registered to dbstl and
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shared across the process.
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</p>
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<p>
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If you are calling the
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<code class="function">dbstl_exit()</code> function, and
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your <code class="classname">DbEnv</code> or
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<code class="classname">Db</code> objects are new'd by
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your code, the <code class="function">dbstl_exit()</code>
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function should be called before deleting the
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<code class="classname">DbEnv</code> or
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<code class="classname">Db</code> objects, because they
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need to be closed before being deleted. Alternatively,
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you can call the
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<code class="methodname">dbstl::close_env()</code> or
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<code class="methodname">dbstl::close_db()</code>
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functions before deleting the
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<code class="classname">DbEnv</code> or
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<code class="classname">Db</code> objects in order to
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explicitly close the databases or environments. If you
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do this, can then delete these objects, and then call
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<code class="function">dbstl_exit()</code>. </p>
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<p>
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In addition, before exiting a thread that uses
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dbstl API, you can call the
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<code class="function">dbstl_thread_exit() </code>function
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to release any Berkeley DB handles if they are not
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used by other threads. If you do not call the
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<code class="function">dbstl_thread_exit() </code>function
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or call this function only in some threads, all open
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Berkeley DB handles will be closed by the
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<code class="function">dbstl_exit()</code>function. You
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must call the <code class="function">dbstl_exit()
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</code>function before the process exits, to avoid
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memory leak and database update loss, if you do not
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have transactions and persistent log files.
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</p>
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</div>
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<div class="sect3" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h4 class="title"><a id="idm140654539118752"></a>DbstlDbt</h4>
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</div>
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</div>
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</div>
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<p>
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Only when you are storing raw bytes (such as a
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bitmap) do you have to store and retrieve data by
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using the <code class="classname">DbstlDbt</code> helper
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class. Although you also can do so simply by using the
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Berkeley DB <code class="classname">Dbt</code> class, the
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<code class="classname">DbstlDbt</code> class offers more
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convenient memory management behavior.
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</p>
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<p>
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When you are storing
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<code class="classname">DbstlDbt</code> objects (such as
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<code class="classname">db_vector<DbstlDbt></code>),
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you <span class="emphasis"><em>must</em></span> allocate heap memory
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explicitly using the <code class="function">malloc()</code>
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function for the <code class="classname">DbstlDbt</code>
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object to reference, but you do not need to free the
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memory – it is automatically freed by the
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<code class="classname">DbstlDbt</code> object that owns
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it by calling the standard C library
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<code class="function">free()</code> function.
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</p>
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<p>
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However, because dbstl supports storing any type of
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object or primitive data, it is rare that you would
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have to store data using
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<code class="classname">DbstlDbt</code> objects while
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using dbstl. Examples of storing
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<code class="classname">DbstlDbt</code> objects can be
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found in the
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<code class="methodname">StlAdvancedFeaturesExample::arbitrary_object_storage()</code>
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and
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<code class="methodname">StlAdvancedFeaturesExample::char_star_string_storage()</code>
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methods.
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</p>
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</div>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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||
<table width="100%" summary="Navigation footer">
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||
<tr>
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||
<td width="40%" align="left"><a accesskey="p" href="stl_efficienct_use.html">Prev</a> </td>
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||
<td width="20%" align="center">
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<a accesskey="u" href="stl.html">Up</a>
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||
</td>
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<td width="40%" align="right"> <a accesskey="n" href="stl_misc.html">Next</a></td>
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||
</tr>
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||
<tr>
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||
<td width="40%" align="left" valign="top">Using dbstl
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efficiently </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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||
</td>
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<td width="40%" align="right" valign="top"> Dbstl miscellaneous notes</td>
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</tr>
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||
</table>
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</div>
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</body>
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</html>
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