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>Db</title> 7 <link rel="stylesheet" href="apiReference.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Berkeley DB C++ API Reference" /> 10 <link rel="up" href="db.html" title="Chapter 2. The Db Handle" /> 11 <link rel="prev" href="dbclose.html" title="Db::close()" /> 12 <link rel="next" href="dbcompact.html" title="Db::compact()" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Db</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="dbclose.html">Prev</a> </td> 22 <th width="60%" align="center">Chapter 2. 23 The Db Handle 24 </th> 25 <td width="20%" align="right"> <a accesskey="n" href="dbcompact.html">Next</a></td> 26 </tr> 27 </table> 28 <hr /> 29 </div> 30 <div class="sect1" lang="en" xml:lang="en"> 31 <div class="titlepage"> 32 <div> 33 <div> 34 <h2 class="title" style="clear: both"><a id="dbcreate"></a>Db</h2> 35 </div> 36 </div> 37 </div> 38 <pre class="programlisting">#include <db_cxx.h> 39 40class Db { 41public: 42 Db(DbEnv *dbenv, u_int32_t flags); 43 ~Db(); 44 45 DB *Db::get_DB(); 46 const DB *Db::get_const_DB() const; 47 static Db *Db::get_Db(DB *db); 48 static const Db *Db::get_const_Db(const DB *db); 49 ... 50}; </pre> 51 <p> 52 The Db handle is the handle for a Berkeley DB database, which may or may not be part of a 53 database environment. 54 </p> 55 <p> 56 Db handles are free-threaded if the 57 <a class="link" href="envopen.html#envopen_DB_THREAD">DB_THREAD</a> flag is specified to 58 the <a class="xref" href="dbopen.html" title="Db::open()">Db::open()</a> method when the database is opened or 59 if the database environment in which the database is opened is free-threaded. The handle 60 should not be closed while any other handle that refers to the database is in use; for 61 example, database handles must not be closed while cursor handles into the database remain 62 open, or transactions that include operations on the database have not yet been committed 63 or aborted. Once the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>, 64 <a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>, 65 <a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a>, or 66 <a class="xref" href="dbverify.html" title="Db::verify()">Db::verify()</a> 67 methods are called, the handle may not be accessed again, regardless of the method's return. 68 </p> 69 <p> 70 The constructor creates a <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> object that is the 71 handle for a Berkeley DB database. The constructor allocates memory internally; calling 72 the <a class="xref" href="dbclose.html" title="Db::close()">Db::close()</a>, 73 <a class="xref" href="dbremove.html" title="Db::remove()">Db::remove()</a>, or 74 <a class="xref" href="dbrename.html" title="Db::rename()">Db::rename()</a> 75 methods will free that memory. 76 </p> 77 <p> 78 Note that destroying the <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> 79 object is synonomous with calling <code class="literal">Db::close(0)</code>. 80 </p> 81 <p> 82 Each <code class="methodname">Db</code> object has an associated <code class="methodname">DB</code> 83 struct, which is used by the underlying implementation of Berkeley DB and its C-language 84 API. The <code class="methodname">Db::get_DB()</code> method returns a pointer to this struct. 85 Given a <code class="methodname">const Db</code> object, 86 <code class="methodname">Db::get_const_DB()</code> returns a const pointer to the same struct. 87 </p> 88 <p> 89 Given a <code class="literal">DB</code> struct, the <code class="methodname">Db::get_Db()</code> method 90 returns the corresponding <code class="classname">Db</code> object, if there is 91 one. If the <code class="literal">DB</code> object was not associated with a 92 <code class="classname">Db</code> (that is, it was not returned from a call 93 to the <code class="methodname">Db::get_DB()</code> method), then the result of 94 <code class="methodname">Db::get_Db()</code> is undefined. Given a 95 <code class="literal">const DB struct</code>, 96 <code class="methodname">Db::get_const_Db()</code> returns the associated 97 <code class="literal">const Db</code>object, if there is one. 98 </p> 99 <p> 100 These methods may be useful for Berkeley DB applications including both C and C++ 101 language software. It should not be necessary to use these calls in a purely C++ 102 application. 103 </p> 104 <div class="sect2" lang="en" xml:lang="en"> 105 <div class="titlepage"> 106 <div> 107 <div> 108 <h3 class="title"><a id="id1631976"></a>Parameters</h3> 109 </div> 110 </div> 111 </div> 112 <div class="sect3" lang="en" xml:lang="en"> 113 <div class="titlepage"> 114 <div> 115 <div> 116 <h4 class="title"><a id="id1632068"></a>dbenv</h4> 117 </div> 118 </div> 119 </div> 120 <p> 121 122 <span>If no <span class="bold"><strong>dbenv</strong></span> value is specified,</span> 123 the database is standalone; that is, it is not part of any Berkeley DB 124 environment. 125 </p> 126 <p> 127 128 <span> 129 If a <span class="bold"><strong>dbenv</strong></span> value is specified, 130 </span> 131 the database is created within the specified Berkeley DB environment. 132 The database access methods automatically make calls to the other 133 subsystems in Berkeley DB, based on the enclosing environment. For 134 example, if the environment has been configured to use locking, the 135 access methods will automatically acquire the correct locks when 136 reading and writing pages of the database. 137 </p> 138 </div> 139 <div class="sect3" lang="en" xml:lang="en"> 140 <div class="titlepage"> 141 <div> 142 <div> 143 <h4 class="title"><a id="id1631995"></a>flags</h4> 144 </div> 145 </div> 146 </div> 147 <p> 148 The <span class="bold"><strong>flags</strong></span> parameter is currently 149 unused, and must be set to 0. 150 </p> 151 <div class="itemizedlist"> 152 <ul type="disc"> 153 <li> 154 <p> 155 <code class="literal">DB_CXX_NO_EXCEPTION</code> 156 </p> 157 <p> 158 The Berkeley DB C++ API supports two different error behaviors. By 159 default, whenever an error occurs, an exception is thrown that 160 encapsulates the error information. This generally allows for 161 cleaner logic for transaction processing because a try block can 162 surround a single transaction. However, if this flag is 163 specified, exceptions are not thrown; instead, each individual 164 function returns an error code. 165 </p> 166 <p> 167 If a <span class="bold"><strong>dbenv</strong></span> value is specified, 168 this flag is ignored, and the error behavior of the specified 169 environment is used instead. 170 </p> 171 </li> 172 </ul> 173 </div> 174 </div> 175 </div> 176 <div class="sect2" lang="en" xml:lang="en"> 177 <div class="titlepage"> 178 <div> 179 <div> 180 <h3 class="title"><a id="id1631996"></a>Class</h3> 181 </div> 182 </div> 183 </div> 184 <p> 185 <a class="link" href="db.html" title="Chapter 2. The Db Handle">Db</a> 186 </p> 187 </div> 188 <div class="sect2" lang="en" xml:lang="en"> 189 <div class="titlepage"> 190 <div> 191 <div> 192 <h3 class="title"><a id="id1632066"></a>See Also</h3> 193 </div> 194 </div> 195 </div> 196 <p> 197 <a class="xref" href="db.html#dblist" title="Database and Related Methods">Database and Related Methods</a> 198 </p> 199 </div> 200 </div> 201 <div class="navfooter"> 202 <hr /> 203 <table width="100%" summary="Navigation footer"> 204 <tr> 205 <td width="40%" align="left"><a accesskey="p" href="dbclose.html">Prev</a> </td> 206 <td width="20%" align="center"> 207 <a accesskey="u" href="db.html">Up</a> 208 </td> 209 <td width="40%" align="right"> <a accesskey="n" href="dbcompact.html">Next</a></td> 210 </tr> 211 <tr> 212 <td width="40%" align="left" valign="top">Db::close() </td> 213 <td width="20%" align="center"> 214 <a accesskey="h" href="index.html">Home</a> 215 </td> 216 <td width="40%" align="right" valign="top"> Db::compact()</td> 217 </tr> 218 </table> 219 </div> 220 </body> 221</html> 222