Edit File: ReplacingEntryWCursor.html
<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Replacing Records Using Cursors</title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> <link rel="home" href="index.html" title="Getting Started with Berkeley DB" /> <link rel="up" href="Cursors.html" title="Chapter 4. Using Cursors" /> <link rel="previous" href="DeleteEntryWCursor.html" title="Deleting Records Using Cursors" /> <link rel="next" href="CoreCursorUsage.html" title="Cursor Example" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Replacing Records Using Cursors</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td> <th width="60%" align="center">Chapter 4. Using Cursors</th> <td width="20%" align="right"> <a accesskey="n" href="CoreCursorUsage.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="ReplacingEntryWCursor"></a>Replacing Records Using Cursors</h2> </div> </div> <div></div> </div> <p> You replace the data for a database record by using <span> <tt class="methodname">Dbc::put()</tt> with the <tt class="literal">DB_CURRENT</tt> flag. </span> </p> <a id="cxx_cursor9"></a> <pre class="programlisting">#include <db_cxx.h> #include <string.h> ... Db my_database(NULL, 0); Dbc *cursorp; int ret; char *key1str = "My first string"; char *replacement_data = "replace me"; try { // Database open omitted // Get the cursor my_database.cursor(NULL, &cursorp, 0); // Set up our DBTs Dbt key(key1str, strlen(key1str) + 1); Dbt data; // Position the cursor */ ret = cursorp->get(&key, &data, DB_SET); if (ret == 0) { data.set_data(replacement_data); data.set_size(strlen(replacement_data) + 1); cursorp->put(&key, &data, DB_CURRENT); } } catch(DbException &e) { my_database.err(e.get_errno(), "Error!"); } catch(std::exception &e) { my_database.errx("Error! %s", e.what()); } // Cursors must be closed if (cursorp != NULL) cursorp->close(); my_database.close(0);</pre> <p> Note that you cannot change a record's key using this method; the key parameter is always ignored when you replace a record. </p> <p> When replacing the data portion of a record, if you are replacing a record that is a member of a sorted duplicates set, then the replacement will be successful only if the new record sorts identically to the old record. This means that if you are replacing a record that is a member of a sorted duplicates set, and if you are using the default lexicographic sort, then the replacement will fail due to violating the sort order. However, if you provide a custom sort routine that, for example, sorts based on just a few bytes out of the data item, then potentially you can perform a direct replacement and still not violate the restrictions described here. </p> <p> <span>Under these circumstances, if</span> you want to replace the data contained by a duplicate record, <span> and you are not using a custom sort routine, then </span> delete the record and create a new record with the desired key and data. </p> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="Cursors.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="CoreCursorUsage.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Deleting Records Using Cursors </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Cursor Example</td> </tr> </table> </div> </body> </html>
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