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<h2 class="title"><a id="databases"></a>Chapter 2. Databases</h2>
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<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="databases.html#DBOpen">Opening Databases</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="coredbclose.html">Closing Databases</a>
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<p>In Berkeley DB, a database is a collection of <span class="emphasis"><em>records</em></span>. Records,
in turn, consist of key/data pairings.
</p>
<p>
Conceptually, you can think of a
<span>database</span>
as containing a two-column table where column 1 contains a key and column 2
contains data. Both the key and the data are managed using
<code class="classname">Dbt</code>
<span>class instances</span>
(see <a class="xref" href="DBEntry.html" title="Chapter 3. Database Records">Database Records</a> for details on this
<span>class</span>
).
So, fundamentally, using a DB
<span>database</span>
involves putting, getting, and deleting database records, which in turns involves efficiently
managing information
<span>encapsulated by </span>
<code class="classname">Dbt</code>
<span>objects.</span>
The next several chapters of this book are dedicated to those activities.
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
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<div>
<h2 class="title" style="clear: both"><a id="DBOpen"></a>Opening Databases</h2>
</div>
</div>
</div>
<p>
You open a database by instantiating a <code class="classname">Db</code> object
and then calling its <code class="methodname">open()</code> method.
</p>
<p>
Note that by default, DB does not create databases if they do not already exist.
To override this behavior, specify the
<a class="link" href="DBOpenFlags.html" title="Database Open Flags"><code class="literal">DB_CREATE</code></a> flag on the
<code class="methodname">open()</code> method.
</p>
<p>
The following code fragment illustrates a database open:
</p>
<a id="cxx_db1"></a>
<pre class="programlisting">#include <db_cxx.h>
...
Db db(NULL, 0); // Instantiate the Db object
u_int32_t oFlags = DB_CREATE; // Open flags;
try {
// Open the database
db.open(NULL, // Transaction pointer
"my_db.db", // Database file name
NULL, // Optional logical database name
DB_BTREE, // Database access method
oFlags, // Open flags
0); // File mode (using defaults)
// DbException is not subclassed from std::exception, so
// need to catch both of these.
} catch(DbException &e) {
// Error handling code goes here
} catch(std::exception &e) {
// Error handling code goes here
} </pre>
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