139 lines
4.9 KiB
HTML
139 lines
4.9 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>Replacing Records Using Cursors</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 V1.62.4" />
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<link rel="home" href="index.html" title="Getting Started with Berkeley DB" />
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<link rel="up" href="Cursors.html" title="Chapter 4. Using Cursors" />
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<link rel="previous" href="DeleteEntryWCursor.html" title="Deleting Records Using Cursors" />
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<link rel="next" href="CoreCursorUsage.html" title="Cursor Example" />
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</head>
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<body>
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Replacing Records Using Cursors</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 4. Using Cursors</th>
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<td width="20%" align="right"> <a accesskey="n" href="CoreCursorUsage.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="ReplacingEntryWCursor"></a>Replacing Records Using Cursors</h2>
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</div>
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</div>
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<div></div>
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</div>
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<p>
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You replace the data for a database record by using
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<span>
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<tt class="methodname">Dbc::put()</tt>
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with the <tt class="literal">DB_CURRENT</tt> flag.
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</span>
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</p>
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<a id="cxx_cursor9"></a>
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<pre class="programlisting">#include <db_cxx.h>
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#include <string.h>
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...
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Db my_database(NULL, 0);
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Dbc *cursorp;
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int ret;
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char *key1str = "My first string";
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char *replacement_data = "replace me";
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try {
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// Database open omitted
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// Get the cursor
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my_database.cursor(NULL, &cursorp, 0);
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// Set up our DBTs
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Dbt key(key1str, strlen(key1str) + 1);
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Dbt data;
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// Position the cursor */
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ret = cursorp->get(&key, &data, DB_SET);
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if (ret == 0) {
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data.set_data(replacement_data);
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data.set_size(strlen(replacement_data) + 1);
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cursorp->put(&key, &data, DB_CURRENT);
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}
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} catch(DbException &e) {
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my_database.err(e.get_errno(), "Error!");
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} catch(std::exception &e) {
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my_database.errx("Error! %s", e.what());
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}
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// Cursors must be closed
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if (cursorp != NULL)
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cursorp->close();
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my_database.close(0);</pre>
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<p>
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Note that you cannot change a record's key using this method; the key
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parameter is always ignored when you replace a record.
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</p>
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<p>
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When replacing the data portion of a record, if you are replacing a
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record that is a member of a sorted duplicates set, then the replacement
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will be successful only if the new record sorts identically to the old
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record. This means that if you are replacing a record that is a member
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of a sorted duplicates set, and if you are using the default
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lexicographic sort, then the replacement will fail due to violating the
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sort order. However, if you
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provide a custom sort routine that, for example, sorts based on just a
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few bytes out of the data item, then potentially you can perform
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a direct replacement and still not violate the restrictions described
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here.
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</p>
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<p>
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<span>Under these circumstances, if</span>
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you want to replace the data contained by a duplicate record,
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<span>
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and you are not using a custom sort routine, then
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</span>
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delete the record and create a new record with the desired key and data.
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</p>
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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="DeleteEntryWCursor.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="Cursors.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="CoreCursorUsage.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">Deleting Records Using Cursors </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"> Cursor Example</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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