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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<td width="20%" align="left"><a accesskey="p" href="stl_efficienct_use.html">Prev</a> </td>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="stl_memory_mgmt"></a>Dbstl memory management</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="stl_memory_mgmt.html#idp952296">Freeing memory</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="stl_memory_mgmt.html#idp994336">Type specific notes</a>
</span>
</dt>
</dl>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp952296"></a>Freeing memory</h3>
</div>
</div>
</div>
<p>
When using dbstl, make sure memory allocated in the
heap is released after use. The rules for this are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
dbstl will free/delete any memory allocated by
dbstl itself.
</p>
</li>
<li>
<p>
You are responsible for freeing/deleting any
memory allocated by your code outside of dbstl.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idp994336"></a>Type specific notes</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp993688"></a>DbEnv/Db</h4>
</div>
</div>
</div>
<p>
When you open a <code class="classname">DbEnv</code> or
<code class="classname">Db</code> object using
<code class="methodname">dbstl::open_env()</code> or
<code class="methodname">dbstl::open_db()</code>, you do
not need to delete that object. However, if you new'd
that object and then opened it without using the
<code class="methodname">dbstl::open_env()</code> or
<code class="methodname">dbstl::open_db()</code> methods,
you are responsible for deleting the object.
</p>
<p>
Note that you must <code class="literal">new</code> the
<code class="classname">Db</code> or
<code class="classname">DbEnv</code> object, which
allocates it on the heap. You can not allocate it on
the stack. If you do, the order of destruction is
uncontrollable, which makes dbstl unable to work
properly.
</p>
<p>
You can call <code class="function">dbstl_exit()</code>
before the process exits, to release any memory
allocated by dbstl that has to live during the entire
process lifetime. Releasing the memory explicitly will
not make much difference, because the process is about
to exit and so all memory allocated on the heap is
going to be returned to the operating system anyway.
The only real difference is that your memory leak
checker will not report false memory leaks.
</p>
<p>
<code class="function">dbstl_exit()</code> releases any memory
allocated by dbstl on the heap. It also performs other
required shutdown operations, such as closing any
databases and environments registered to dbstl and
shared across the process.
</p>
<p>
If you are calling the
<code class="function">dbstl_exit()</code> function, and
your <code class="classname">DbEnv</code> or
<code class="classname">Db</code> objects are new'd by
your code, the <code class="function">dbstl_exit()</code>
function should be called before deleting the
<code class="classname">DbEnv</code> or
<code class="classname">Db</code> objects, because they
need to be closed before being deleted. Alternatively,
you can call the
<code class="methodname">dbstl::close_env()</code> or
<code class="methodname">dbstl::close_db()</code>
functions before deleting the
<code class="classname">DbEnv</code> or
<code class="classname">Db</code> objects in order to
explicitly close the databases or environments. If you
do this, can then delete these objects, and then call
<code class="function">dbstl_exit()</code>. </p>
<p>
In addition, before exiting a thread that uses
dbstl API, you can call the
<code class="function">dbstl_thread_exit() </code>function
to release any Berkeley DB handles if they are not
used by other threads. If you do not call the
<code class="function">dbstl_thread_exit() </code>function
or call this function only in some threads, all open
Berkeley DB handles will be closed by the
<code class="function">dbstl_exit()</code>function. You
must call the <code class="function">dbstl_exit()
</code>function before the process exits, to avoid
memory leak and database update loss, if you do not
have transactions and persistent log files.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp953376"></a>DbstlDbt</h4>
</div>
</div>
</div>
<p>
Only when you are storing raw bytes (such as a
bitmap) do you have to store and retrieve data by
using the <code class="classname">DbstlDbt</code> helper
class. Although you also can do so simply by using the
Berkeley DB <code class="classname">Dbt</code> class, the
<code class="classname">DbstlDbt</code> class offers more
convenient memory management behavior.
</p>
<p>
When you are storing
<code class="classname">DbstlDbt</code> objects (such as
<code class="classname">db_vector<DbstlDbt></code>),
you <span class="emphasis"><em>must</em></span> allocate heap memory
explicitly using the <code class="function">malloc()</code>
function for the <code class="classname">DbstlDbt</code>
object to reference, but you do not need to free the
memory – it is automatically freed by the
<code class="classname">DbstlDbt</code> object that owns
it by calling the standard C library
<code class="function">free()</code> function.
</p>
<p>
However, because dbstl supports storing any type of
object or primitive data, it is rare that you would
have to store data using
<code class="classname">DbstlDbt</code> objects while
using dbstl. Examples of storing
<code class="classname">DbstlDbt</code> objects can be
found in the
<code class="methodname">StlAdvancedFeaturesExample::arbitrary_object_storage()</code>
and
<code class="methodname">StlAdvancedFeaturesExample::char_star_string_storage()</code>
methods.
</p>
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