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<th width="60%" align="center">Chapter 2.
The Db Handle
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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="dbcreate"></a>Db</h2>
</div>
</div>
</div>
<pre class="programlisting">#include <db_cxx.h>
class Db {
public:
Db(DbEnv *dbenv, u_int32_t flags);
~Db();
DB *Db::get_DB();
const DB *Db::get_const_DB() const;
static Db *Db::get_Db(DB *db);
static const Db *Db::get_const_Db(const DB *db);
...
}; </pre>
<p>
The Db handle is the handle for a Berkeley DB database, which may or may not be part of a
database environment.
</p>
<p>
Db handles are free-threaded if the
<a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a> flag is specified to
the <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method when the database is opened or
if the database environment in which the database is opened is free-threaded. The handle
should not be closed while any other handle that refers to the database is in use; for
example, database handles must not be closed while cursor handles into the database remain
open, or transactions that include operations on the database have not yet been committed
or aborted. Once the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>,
<a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>,
<a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>, or
<a class="xref" href="dbverify.html" title="Db::verify()">Db::verify()</a>
methods are called, the handle may not be accessed again, regardless of the method's return.
</p>
<p>
The constructor creates a <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> object that is the
handle for a Berkeley DB database. The constructor allocates memory internally; calling
the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>,
<a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>, or
<a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>
methods will free that memory.
</p>
<p>
Note that destroying the <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
object is synonomous with calling <code class="literal">Db::close(0)</code>.
</p>
<p>
Each <code class="methodname">Db</code> object has an associated <code class="methodname">DB</code>
struct, which is used by the underlying implementation of Berkeley DB and its C-language
API. The <code class="methodname">Db::get_DB()</code> method returns a pointer to this struct.
Given a <code class="methodname">const Db</code> object,
<code class="methodname">Db::get_const_DB()</code> returns a const pointer to the same struct.
</p>
<p>
Given a <code class="literal">DB</code> struct, the <code class="methodname">Db::get_Db()</code> method
returns the corresponding <code class="classname">Db</code> object, if there is
one. If the <code class="literal">DB</code> object was not associated with a
<code class="classname">Db</code> (that is, it was not returned from a call
to the <code class="methodname">Db::get_DB()</code> method), then the result of
<code class="methodname">Db::get_Db()</code> is undefined. Given a
<code class="literal">const DB struct</code>,
<code class="methodname">Db::get_const_Db()</code> returns the associated
<code class="literal">const Db</code>object, if there is one.
</p>
<p>
These methods may be useful for Berkeley DB applications including both C and C++
language software. It should not be necessary to use these calls in a purely C++
application.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm50360"></a>Parameters</h3>
</div>
</div>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idp88024"></a>dbenv</h4>
</div>
</div>
</div>
<p>
<span>If no <span class="bold"><strong>dbenv</strong></span> value is specified,</span>
the database is standalone; that is, it is not part of any Berkeley DB
environment.
</p>
<p>
<span>
If a <span class="bold"><strong>dbenv</strong></span> value is specified,
</span>
the database is created within the specified Berkeley DB environment.
The database access methods automatically make calls to the other
subsystems in Berkeley DB, based on the enclosing environment. For
example, if the environment has been configured to use locking, the
access methods will automatically acquire the correct locks when
reading and writing pages of the database.
</p>
</div>
<div class="sect3" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h4 class="title"><a id="idm357600"></a>flags</h4>
</div>
</div>
</div>
<p>
The <span class="bold"><strong>flags</strong></span> parameter must be set to 0 or the following value:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">DB_CXX_NO_EXCEPTION</code>
</p>
<p>
The Berkeley DB C++ API supports two different error behaviors. By
default, whenever an error occurs, an exception is thrown that
encapsulates the error information. This generally allows for
cleaner logic for transaction processing because a try block can
surround a single transaction. However, if this flag is
specified, exceptions are not thrown; instead, each individual
function returns an error code.
</p>
<p>
If a <span class="bold"><strong>dbenv</strong></span> value is specified,
this flag is ignored, and the error behavior of the specified
environment is used instead.
</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm1691064"></a>Class</h3>
</div>
</div>
</div>
<p>
<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="idm1672480"></a>See Also</h3>
</div>
</div>
</div>
<p>
<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
</p>
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