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<html>
<head>
<meta http-equiv="Content-Type" content=
"text/html; charset=iso-8859-1">
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"threads, Boost.Threads, thread library, C++">
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<title>Boost.Threads, thread_specific_ptr</title>
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<table summary="header" border="0" cellpadding="7" cellspacing="0"
width="100%">
<tr>
<td valign="top" width="300">
<h3><img src="../../../c++boost.gif" alt="C++ Boost" width=
"277" height="86"></h3>
</td>
<td valign="top">
<h1 align="center">Boost.Threads</h1>
<h2 align="center">thread_specific_ptr</h2>
</td>
</tr>
</table>
<hr>
<p><a href="#Introduction">Introduction</a><br>
<a href="#Header">Header</a><br>
<a href="#Synopsis">Synopsis</a><br>
<a href="#Members">Members</a><br>
<a href="#Example">Example</a></p>
<h2><a name="Introduction">Introduction</a></h2>
<p>The <code>thread_specific_ptr</code> class defines an interface for
using thread specific storage. Thread specific storage is data
associated with individual threads and is often used to make operations
<a href="definitions.html#Thread-safe">thread-safe</a> that rely on
global data.</p>
<p>Template <code>thread_specific_ptr</code> stores a pointer to an
object obtained via <code>new</code> on a thread-by-thread basis and
calls delete on the contained pointer when the thread terminates. Each
thread initially stores the null pointer in each <code>
thread_specific_ptr</code> instance.</p>
<p>The template <code>thread_specific_ptr</code> is useful in the
following cases:</p>
<ul>
<li>An interface was original written assuming a single thread of
control and is being ported to a multi-threaded environment.</li>
<li>Each thread of control invokes sequences of methods that share
data that must be logically accessed through a globally visible
access point, but are physically unique for each thread, instead of
being explicitly passed.</li>
</ul>
<h2><a name="Header">Header</a></h2>
<pre>
#include <a href=
"../../../boost/thread/tss.hpp"><boost/thread/tss.hpp></a>
</pre>
<h2><a name="Synopsis">Synopsis</a></h2>
<pre>
namespace boost {
template <typename T>
class thread_specific_ptr : private boost::noncopyable // Exposition only.
// Class thread_specific_ptr meets the <a href=
"overview.html#NonCopyable">NonCopyable</a> requirement.
{
public:
thread_specific_ptr();
~thread_specific_ptr();
T* get() const;
T* operator->() const;
T& operator*() const;
T* release();
void reset(T* p=0);
};
} // namespace boost
</pre>
<h2><a name="Members">Members</a></h2>
<hr>
<h3>Constructor</h3>
<pre>
thread_specific_ptr();
</pre>
<p><b>Postconditions:</b> A thread specific storage has been reserved
for use by *this in all threads, with each thread initially storing a
null pointer.</p>
<p><b>Requires:</b> The expression <code>delete get()</code> is well
formed.</p>
<p><b>Throws:</b> <code>boost::thread_resource_error</code> if the
necessary resources can not be obtained.</p>
<p><b>Notes:</b> There is an implementation specific limit to the
number of thread specific storage objects that can be created, and this
limit may be small.</p>
<hr>
<h3>Destructor</h3>
<pre>
~thread_specific_ptr();
</pre>
<p><b>Notes:</b> Does not destroy any data that may be stored in any
thread's thread specific storage. For this reason you should not
destroy a <code>thread_specific_ptr</code> object until you are certain
there are no threads running that have made use of its thread specific
storage.</p>
<hr>
<h3>get</h3>
<pre>
T* get() const;
</pre>
<p><b>Returns:</b> The object stored in thread specific storage for the
current thread for *this.</p>
<p><b>Notes:</b> Each thread initially returns 0.</p>
<hr>
<h3>Smart Pointer Operations</h3>
<pre>
T* operator->() const;
</pre>
<p><b>Returns:</b> <code>get()</code></p>
<pre>
T& operator*() const;
</pre>
<p><b>Returns:</b> <code>get()</code></p>
<p><b>Requires:</b> <code>get() != 0</code></p>
<hr>
<h3>Release</h3>
<pre>
T* release();
</pre>
<p><b>Returns:</b> <code>get()</code></p>
<p><b>Postcondition:</b> *this holds the null pointer for the current
thread.</p>
<hr>
<h3>Reset</h3>
<pre>
void reset(T* p=0);
</pre>
<p><b>Effects:</b> If <code>get()!= p</code> then <code>delete
get()</code>.</p>
<p><b>Postconditions:</b> <code>*this</code> holds the pointer <code>
p</code> for the current thread.</p>
<p><b>Notes:</b> The pointer will be deleted when the thread
terminates.</p>
<hr>
<h2><a name="Example">Example Usage</a></h2>
<pre>
#include <a href=
"../../../boost/thread/thread.hpp"><boost/thread/thread.hpp></a>
#include <a href=
"../../../boost/thread/tss.hpp"><boost/thread/tss.hpp></a>
#include <cassert>
boost::thread_specific_ptr<int> value;
void increment()
{
int* p = value.get();
++*p;
}
void thread_proc()
{
value.reset(new int(0)); // initialize the thread's storage
for (int i=0; i<10; ++i)
{
increment();
int* p = value.get();
assert(*p == i+1);
}
}
int main(int argc, char* argv[])
{
boost::thread_group threads;
for (int i=0; i<5; ++i)
threads.create_thread(&thread_proc);
threads.join_all();
}
</pre>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->05 November, 2001<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
<p><i>© Copyright <a href="mailto:williamkempf@hotmail.com">
William E. Kempf</a> 2001 all rights reserved.</i></p>
</body>
</html>
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