source: trunk/doc/html/qmutexlocker.html

Last change on this file was 190, checked in by rudi, 14 years ago

reference documentation added
File size: 7.5 KB
Line 
1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
2<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/src/tools/qmutex_unix.cpp:511 -->
3<html>
4<head>
5<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
6<title>QMutexLocker Class</title>
7<style type="text/css"><!--
8fn { margin-left: 1cm; text-indent: -1cm; }
9a:link { color: #004faf; text-decoration: none }
10a:visited { color: #672967; text-decoration: none }
11body { background: #ffffff; color: black; }
12--></style>
13</head>
14<body>
15
16<table border="0" cellpadding="0" cellspacing="0" width="100%">
17<tr bgcolor="#E5E5E5">
18<td valign=center>
19 <a href="index.html">
20<font color="#004faf">Home</font></a>
21 | <a href="classes.html">
22<font color="#004faf">All&nbsp;Classes</font></a>
23 | <a href="mainclasses.html">
24<font color="#004faf">Main&nbsp;Classes</font></a>
25 | <a href="annotated.html">
26<font color="#004faf">Annotated</font></a>
27 | <a href="groups.html">
28<font color="#004faf">Grouped&nbsp;Classes</font></a>
29 | <a href="functions.html">
30<font color="#004faf">Functions</font></a>
31</td>
32<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>QMutexLocker Class Reference</h1>
33
34<p>The QMutexLocker class simplifies locking and unlocking QMutexes.
35<a href="#details">More...</a>
36<p>All the functions in this class are <a href="threads.html#threadsafe">thread-safe</a> when Qt is built with thread support.</p>
37<p><tt>#include &lt;<a href="qmutex-h.html">qmutex.h</a>&gt;</tt>
38<p><a href="qmutexlocker-members.html">List of all member functions.</a>
39<h2>Public Members</h2>
40<ul>
41<li class=fn><a href="#QMutexLocker"><b>QMutexLocker</b></a> ( QMutex&nbsp;*&nbsp;mutex )</li>
42<li class=fn><a href="#~QMutexLocker"><b>~QMutexLocker</b></a> ()</li>
43<li class=fn>QMutex * <a href="#mutex"><b>mutex</b></a> () const</li>
44</ul>
45<hr><a name="details"></a><h2>Detailed Description</h2>
46
47
48The QMutexLocker class simplifies locking and unlocking QMutexes.
49<p>
50<p>
51
52<p> The purpose of QMutexLocker is to simplify <a href="qmutex.html">QMutex</a> locking and
53unlocking. Locking and unlocking a QMutex in complex functions and
54statements or in exception handling code is error prone and
55difficult to debug. QMutexLocker should be used in such situations
56to ensure that the state of the mutex is well defined and always
57locked and unlocked properly.
58<p> QMutexLocker should be created within a function where a QMutex
59needs to be locked. The mutex is locked when QMutexLocker is
60created, and unlocked when QMutexLocker is destroyed.
61<p> For example, this complex function locks a QMutex upon entering
62the function and unlocks the mutex at all the exit points:
63<p> <pre>
64 int complexFunction( int flag )
65 {
66 mutex.lock();
67
68 int return_value = 0;
69
70 switch ( flag ) {
71 case 0:
72 case 1:
73 {
74 mutex.unlock();
75 return moreComplexFunction( flag );
76 }
77
78 case 2:
79 {
80 int status = anotherFunction();
81 if ( status &lt; 0 ) {
82 mutex.unlock();
83 return -2;
84 }
85 return_value = status + flag;
86 break;
87 }
88
89 default:
90 {
91 if ( flag &gt; 10 ) {
92 mutex.unlock();
93 return -1;
94 }
95 break;
96 }
97 }
98
99 mutex.unlock();
100 return return_value;
101 }
102 </pre>
103
104<p> This example function will get more complicated as it is
105developed, which increases the likelihood that errors will occur.
106<p> Using QMutexLocker greatly simplifies the code, and makes it more
107readable:
108<p> <pre>
109 int complexFunction( int flag )
110 {
111 QMutexLocker locker( &amp;mutex );
112
113 int return_value = 0;
114
115 switch ( flag ) {
116 case 0:
117 case 1:
118 {
119 return moreComplexFunction( flag );
120 }
121
122 case 2:
123 {
124 int status = anotherFunction();
125 if ( status &lt; 0 )
126 return -2;
127 return_value = status + flag;
128 break;
129 }
130
131 default:
132 {
133 if ( flag &gt; 10 )
134 return -1;
135 break;
136 }
137 }
138
139 return return_value;
140 }
141 </pre>
142
143<p> Now, the mutex will always be unlocked when the QMutexLocker
144object is destroyed (when the function returns since <tt>locker</tt> is
145an auto variable). Note that the mutex will be unlocked after
146the call to moreComplexFunction() in this example, avoiding
147possible bugs caused by unlocking the mutex too early, as in
148the first example.
149<p> The same principle applies to code that throws and catches
150exceptions. An exception that is not caught in the function that
151has locked the mutex has no way of unlocking the mutex before the
152exception is passed up the stack to the calling function.
153<p> QMutexLocker also provides a <a href="#mutex">mutex</a>() member function that returns
154the mutex on which the QMutexLocker is operating. This is useful
155for code that needs access to the mutex, such as
156<a href="qwaitcondition.html#wait">QWaitCondition::wait</a>(). For example:
157<p> <pre>
158 class SignalWaiter
159 {
160 private:
161 QMutexLocker locker;
162
163 public:
164 SignalWaiter( <a href="qmutex.html">QMutex</a> *mutex )
165 : locker( mutex )
166 {
167 }
168
169 void waitForSignal()
170 {
171 ...
172 ...
173 ...
174
175 while ( ! signalled )
176 waitcondition.wait( locker.<a href="#mutex">mutex</a>() );
177
178 ...
179 ...
180 ...
181 }
182 };
183 </pre>
184
185<p> <p>See also <a href="qmutex.html">QMutex</a>, <a href="qwaitcondition.html">QWaitCondition</a>, <a href="environment.html">Environment Classes</a>, and <a href="thread.html">Threading</a>.
186
187<hr><h2>Member Function Documentation</h2>
188<h3 class=fn><a name="QMutexLocker"></a>QMutexLocker::QMutexLocker ( <a href="qmutex.html">QMutex</a>&nbsp;*&nbsp;mutex )
189</h3>
190
191<p> Constructs a QMutexLocker and locks <em>mutex</em>. The mutex will be
192unlocked when the QMutexLocker is destroyed. If <em>mutex</em> is zero,
193QMutexLocker does nothing.
194<p> <p>See also <a href="qmutex.html#lock">QMutex::lock</a>().
195
196<h3 class=fn><a name="~QMutexLocker"></a>QMutexLocker::~QMutexLocker ()
197</h3>
198
199<p> Destroys the QMutexLocker and unlocks the mutex which was locked
200in the constructor.
201<p> <p>See also <a href="#QMutexLocker">QMutexLocker::QMutexLocker</a>() and <a href="qmutex.html#unlock">QMutex::unlock</a>().
202
203<h3 class=fn><a href="qmutex.html">QMutex</a>&nbsp;* <a name="mutex"></a>QMutexLocker::mutex () const
204</h3>
205
206<p> Returns a pointer to the mutex which was locked in the
207constructor.
208<p> <p>See also <a href="#QMutexLocker">QMutexLocker::QMutexLocker</a>().
209
210<!-- eof -->
211<hr><p>
212This file is part of the <a href="index.html">Qt toolkit</a>.
213Copyright &copy; 1995-2007
214<a href="http://www.trolltech.com/">Trolltech</a>. All Rights Reserved.<p><address><hr><div align=center>
215<table width=100% cellspacing=0 border=0><tr>
216<td>Copyright &copy; 2007
217<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
218<td align=right><div align=right>Qt 3.3.8</div>
219</table></div></address></body>
220</html>
Note: See TracBrowser for help on using the repository browser.