1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Replacing Records Using Cursors</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> 9 <link rel="home" href="index.html" title="Getting Started with Berkeley DB" /> 10 <link rel="up" href="Cursors.html" title="Chapter 4. Using Cursors" /> 11 <link rel="previous" href="DeleteEntryWCursor.html" title="Deleting Records Using Cursors" /> 12 <link rel="next" href="CoreCursorUsage.html" title="Cursor Example" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Replacing Records Using Cursors</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td> 22 <th width="60%" align="center">Chapter 4. Using Cursors</th> 23 <td width="20%" align="right"> <a accesskey="n" href="CoreCursorUsage.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="ReplacingEntryWCursor"></a>Replacing Records Using Cursors</h2> 33 </div> 34 </div> 35 <div></div> 36 </div> 37 <p> 38 You replace the data for a database record by using 39 40 41 42 43 <span> 44 45 <tt class="methodname">Dbc::put()</tt> 46 with the <tt class="literal">DB_CURRENT</tt> flag. 47 </span> 48 49 </p> 50 <a id="cxx_cursor9"></a> 51 <pre class="programlisting">#include <db_cxx.h> 52#include <string.h> 53 54... 55 56Db my_database(NULL, 0); 57Dbc *cursorp; 58 59int ret; 60char *key1str = "My first string"; 61char *replacement_data = "replace me"; 62 63try { 64 // Database open omitted 65 66 // Get the cursor 67 my_database.cursor(NULL, &cursorp, 0); 68 69 // Set up our DBTs 70 Dbt key(key1str, strlen(key1str) + 1); 71 Dbt data; 72 73 // Position the cursor */ 74 ret = cursorp->get(&key, &data, DB_SET); 75 if (ret == 0) { 76 data.set_data(replacement_data); 77 data.set_size(strlen(replacement_data) + 1); 78 cursorp->put(&key, &data, DB_CURRENT); 79 } 80} catch(DbException &e) { 81 my_database.err(e.get_errno(), "Error!"); 82} catch(std::exception &e) { 83 my_database.errx("Error! %s", e.what()); 84} 85 86// Cursors must be closed 87if (cursorp != NULL) 88 cursorp->close(); 89 90my_database.close(0);</pre> 91 <p> 92 Note that you cannot change a record's key using this method; the key 93 parameter is always ignored when you replace a record. 94 </p> 95 <p> 96 When replacing the data portion of a record, if you are replacing a 97 record that is a member of a sorted duplicates set, then the replacement 98 will be successful only if the new record sorts identically to the old 99 record. This means that if you are replacing a record that is a member 100 of a sorted duplicates set, and if you are using the default 101 lexicographic sort, then the replacement will fail due to violating the 102 sort order. However, if you 103 provide a custom sort routine that, for example, sorts based on just a 104 few bytes out of the data item, then potentially you can perform 105 a direct replacement and still not violate the restrictions described 106 here. 107 </p> 108 <p> 109 <span>Under these circumstances, if</span> 110 111 you want to replace the data contained by a duplicate record, 112 <span> 113 and you are not using a custom sort routine, then 114 </span> 115 delete the record and create a new record with the desired key and data. 116 </p> 117 </div> 118 <div class="navfooter"> 119 <hr /> 120 <table width="100%" summary="Navigation footer"> 121 <tr> 122 <td width="40%" align="left"><a accesskey="p" href="DeleteEntryWCursor.html">Prev</a> </td> 123 <td width="20%" align="center"> 124 <a accesskey="u" href="Cursors.html">Up</a> 125 </td> 126 <td width="40%" align="right"> <a accesskey="n" href="CoreCursorUsage.html">Next</a></td> 127 </tr> 128 <tr> 129 <td width="40%" align="left" valign="top">Deleting Records Using Cursors </td> 130 <td width="20%" align="center"> 131 <a accesskey="h" href="index.html">Home</a> 132 </td> 133 <td width="40%" align="right" valign="top"> Cursor Example</td> 134 </tr> 135 </table> 136 </div> 137 </body> 138</html> 139