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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>QThreadStorage Class Reference</h1>
<p>The QThreadStorage class provides per-thread data storage.
<a href="#details">More...</a>
<p>All the functions in this class are <a href="threads.html#threadsafe">thread-safe</a> when Qt is built with thread support.</p>
<p><tt>#include <<a href="qthreadstorage-h.html">qthreadstorage.h</a>></tt>
<p><a href="qthreadstorage-members.html">List of all member functions.</a>
<h2>Public Members</h2>
<ul>
<li class=fn><a href="#QThreadStorage"><b>QThreadStorage</b></a> ()</li>
<li class=fn><a href="#~QThreadStorage"><b>~QThreadStorage</b></a> ()</li>
<li class=fn>bool <a href="#hasLocalData"><b>hasLocalData</b></a> () const</li>
<li class=fn>T & <a href="#localData"><b>localData</b></a> ()</li>
<li class=fn>T <a href="#localData-2"><b>localData</b></a> () const</li>
<li class=fn>void <a href="#setLocalData"><b>setLocalData</b></a> ( T data )</li>
</ul>
<hr><a name="details"></a><h2>Detailed Description</h2>
The QThreadStorage class provides per-thread data storage.
<p>
<p> QThreadStorage is a template class that provides per-thread data
storage.
<p> <em>Note that due to compiler limitations, QThreadStorage can only store pointers.</em>
<p> The <a href="#setLocalData">setLocalData</a>() function stores a single thread-specific value
for the calling thread. The data can be accessed later using the
<a href="#localData">localData</a>() functions. QThreadStorage takes ownership of the
data (which must be created on the heap with <em>new</em>) and deletes
it when the thread exits (either normally or via termination).
<p> The <a href="#hasLocalData">hasLocalData</a>() function allows the programmer to determine if
data has previously been set using the setLocalData() function.
This is useful for lazy initializiation.
<p> For example, the following code uses QThreadStorage to store a
single cache for each thread that calls the <em>cacheObject()</em> and
<em>removeFromCache()</em> functions. The cache is automatically
deleted when the calling thread exits (either normally or via
termination).
<p> <pre>
QThreadStorage<QCache<SomeClass> *> caches;
void cacheObject( const <a href="qstring.html">QString</a> &key, SomeClass *object )
{
if ( ! caches.hasLocalData() )
caches.setLocalData( new <a href="qcache.html">QCache</a><SomeClass> );
caches.localData()->insert( key, object );
}
void removeFromCache( const <a href="qstring.html">QString</a> &key )
{
if ( ! caches.hasLocalData() )
return; // nothing to do
caches.localData()->remove( key );
}
</pre>
<p> <h3> Caveats
</h3>
<a name="1"></a><p> <ul>
<p> <li> As noted above, QThreadStorage can only store pointers due to
compiler limitations. Support for value-based objects will be
added when the majority of compilers are able to support partial
template specialization.
<p> <li> The <a href="#~QThreadStorage">destructor</a> does <em>not</em>
delete per-thread data. QThreadStorage only deletes per-thread
data when the thread exits or when <a href="#setLocalData">setLocalData</a>() is called
multiple times.
<p> <li> QThreadStorage can only be used with threads started with
<a href="qthread.html">QThread</a>. It <em>cannot</em> be used with threads started with
platform-specific APIs.
<p> <li> As a corollary to the above, platform-specific APIs cannot be
used to exit or terminate a QThread using QThreadStorage. Doing so
will cause all per-thread data to be leaked. See <a href="qthread.html#exit">QThread::exit</a>()
and <a href="qthread.html#terminate">QThread::terminate</a>().
<p> <li> QThreadStorage <em>can</em> be used to store data for the <em>main()</em>
thread <em>after</em> <a href="qapplication.html">QApplication</a> has been constructed. QThreadStorage
deletes all data set for the <em>main()</em> thread when QApplication is
destroyed, regardless of whether or not the <em>main()</em> thread has
actually finished.
<p> <li> The implementation of QThreadStorage limits the total number of
QThreadStorage objects to 256. An unlimited number of threads
can store per-thread data in each QThreadStorage object.
<p> </ul>
<p>See also <a href="environment.html">Environment Classes</a> and <a href="thread.html">Threading</a>.
<hr><h2>Member Function Documentation</h2>
<h3 class=fn><a name="QThreadStorage"></a>QThreadStorage::QThreadStorage ()
</h3>
<p> Constructs a new per-thread data storage object.
<h3 class=fn><a name="~QThreadStorage"></a>QThreadStorage::~QThreadStorage ()
</h3>
<p> Destroys the per-thread data storage object.
<p> Note: The per-thread data stored is <em>not</em> deleted. Any data left
in QThreadStorage is leaked. Make sure that all threads using
QThreadStorage have exited before deleting the QThreadStorage.
<p> <p>See also <a href="#hasLocalData">hasLocalData</a>().
<h3 class=fn>bool <a name="hasLocalData"></a>QThreadStorage::hasLocalData () const
</h3>
<p> Returns TRUE if the calling thread has non-zero data available;
otherwise returns FALSE.
<p> <p>See also <a href="#localData">localData</a>().
<h3 class=fn>T & <a name="localData"></a>QThreadStorage::localData ()
</h3>
<p> Returns a reference to the data that was set by the calling
thread.
<p> Note: QThreadStorage can only store pointers. This function
returns a <em>reference</em> to the pointer that was set by the calling
thread. The value of this reference is 0 if no data was set by
the calling thread,
<p> <p>See also <a href="#hasLocalData">hasLocalData</a>().
<h3 class=fn>T <a name="localData-2"></a>QThreadStorage::localData () const
</h3>
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
<p> Returns a copy of the data that was set by the calling thread.
<p> Note: QThreadStorage can only store pointers. This function
returns a pointer to the data that was set by the calling thread.
If no data was set by the calling thread, this function returns 0.
<p> <p>See also <a href="#hasLocalData">hasLocalData</a>().
<h3 class=fn>void <a name="setLocalData"></a>QThreadStorage::setLocalData ( T data )
</h3>
<p> Sets the local data for the calling thread to <em>data</em>. It can be
accessed later using the <a href="#localData">localData</a>() functions.
<p> If <em>data</em> is 0, this function deletes the previous data (if
any) and returns immediately.
<p> If <em>data</em> is non-zero, QThreadStorage takes ownership of the <em>data</em> and deletes it automatically either when the thread exits
(either normally or via termination) or when <a href="#setLocalData">setLocalData</a>() is
called again.
<p> Note: QThreadStorage can only store pointers. The <em>data</em>
argument must be either a pointer to an object created on the heap
(i.e. using <em>new</em>) or 0. You should not delete <em>data</em>
yourself; QThreadStorage takes ownership and will delete the <em>data</em> itself.
<p> <p>See also <a href="#localData">localData</a>() and <a href="#hasLocalData">hasLocalData</a>().
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