2011-09-13 17:44:24 +00:00
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<title>Db</title>
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<link rel="start" href="index.html" title="Berkeley DB C++ API Reference" />
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<link rel="up" href="db.html" title="Chapter 2. The Db Handle" />
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<link rel="prev" href="db_copy.html" title="db_copy" />
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<p>Library Version 11.2.5.2</p>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Db</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="db_copy.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 2.
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The Db Handle
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="dbdel.html">Next</a></td>
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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="dbcreate"></a>Db</h2>
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</div>
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</div>
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</div>
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<pre class="programlisting">#include <db_cxx.h>
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class Db {
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public:
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Db(DbEnv *dbenv, u_int32_t flags);
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~Db();
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DB *Db::get_DB();
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const DB *Db::get_const_DB() const;
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static Db *Db::get_Db(DB *db);
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static const Db *Db::get_const_Db(const DB *db);
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...
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}; </pre>
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<p>
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The Db handle is the handle for a Berkeley DB database, which may or may not be part of a
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database environment.
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</p>
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<p>
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Db handles are free-threaded if the
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<a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a> flag is specified to
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the <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method when the database is opened or
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if the database environment in which the database is opened is free-threaded. The handle
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should not be closed while any other handle that refers to the database is in use; for
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example, database handles must not be closed while cursor handles into the database remain
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open, or transactions that include operations on the database have not yet been committed
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or aborted. Once the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>,
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<a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>,
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<a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>, or
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<a class="xref" href="dbverify.html" title="Db::verify()">Db::verify()</a>
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methods are called, the handle may not be accessed again, regardless of the method's return.
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</p>
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<p>
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The constructor creates a <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> object that is the
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handle for a Berkeley DB database. The constructor allocates memory internally; calling
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the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>,
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<a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>, or
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<a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>
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methods will free that memory.
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</p>
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<p>
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Note that destroying the <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
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object is synonomous with calling <code class="literal">Db::close(0)</code>.
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</p>
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<p>
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Each <code class="methodname">Db</code> object has an associated <code class="methodname">DB</code>
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struct, which is used by the underlying implementation of Berkeley DB and its C-language
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API. The <code class="methodname">Db::get_DB()</code> method returns a pointer to this struct.
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Given a <code class="methodname">const Db</code> object,
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<code class="methodname">Db::get_const_DB()</code> returns a const pointer to the same struct.
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</p>
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<p>
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Given a <code class="literal">DB</code> struct, the <code class="methodname">Db::get_Db()</code> method
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returns the corresponding <code class="classname">Db</code> object, if there is
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one. If the <code class="literal">DB</code> object was not associated with a
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<code class="classname">Db</code> (that is, it was not returned from a call
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to the <code class="methodname">Db::get_DB()</code> method), then the result of
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<code class="methodname">Db::get_Db()</code> is undefined. Given a
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<code class="literal">const DB struct</code>,
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<code class="methodname">Db::get_const_Db()</code> returns the associated
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<code class="literal">const Db</code>object, if there is one.
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</p>
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<p>
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These methods may be useful for Berkeley DB applications including both C and C++
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language software. It should not be necessary to use these calls in a purely C++
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application.
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</p>
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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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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3560920"></a>Parameters</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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2011-12-20 00:07:10 +00:00
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<h4 class="title"><a id="id3560924"></a>dbenv</h4>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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<span>If no <span class="bold"><strong>dbenv</strong></span> value is specified,</span>
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the database is standalone; that is, it is not part of any Berkeley DB
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environment.
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</p>
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<p>
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<span>
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If a <span class="bold"><strong>dbenv</strong></span> value is specified,
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</span>
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the database is created within the specified Berkeley DB environment.
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The database access methods automatically make calls to the other
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subsystems in Berkeley DB, based on the enclosing environment. For
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example, if the environment has been configured to use locking, the
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access methods will automatically acquire the correct locks when
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reading and writing pages of the database.
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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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2011-12-20 00:07:10 +00:00
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<h4 class="title"><a id="id3560958"></a>flags</h4>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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The <span class="bold"><strong>flags</strong></span> parameter must be set to 0 or the following value:
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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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<code class="literal">DB_CXX_NO_EXCEPTION</code>
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</p>
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<p>
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The Berkeley DB C++ API supports two different error behaviors. By
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default, whenever an error occurs, an exception is thrown that
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encapsulates the error information. This generally allows for
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cleaner logic for transaction processing because a try block can
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surround a single transaction. However, if this flag is
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specified, exceptions are not thrown; instead, each individual
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function returns an error code.
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</p>
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<p>
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If a <span class="bold"><strong>dbenv</strong></span> value is specified,
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this flag is ignored, and the error behavior of the specified
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environment is used instead.
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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>
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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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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3560953"></a>Class</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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<a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a>
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</p>
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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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2011-12-20 00:07:10 +00:00
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<h3 class="title"><a id="id3561051"></a>See Also</h3>
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2011-09-13 17:44:24 +00:00
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</div>
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</div>
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</div>
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<p>
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<a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a>
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</p>
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<a accesskey="u" href="db.html">Up</a>
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<td width="40%" align="right"> <a accesskey="n" href="dbdel.html">Next</a></td>
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<td width="40%" align="left" valign="top">db_copy </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"> Db::del()</td>
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</tr>
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</table>
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