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<!-- Copyright (c) 2002-2003 William E. Kempf, Michael Glassford
     Subject to the Boost Software License, Version 1.0. 
     (See accompanying file LICENSE-1.0 or  http://www.boost.org/LICENSE-1.0)
-->
<header name="boost/thread/tss.hpp"
	last-revision="$Date: 2006/10/15 14:52:54 $">
	<namespace name="boost">
		<class name="thread_specific_ptr">
			<purpose>
				The <classname>thread_specific_ptr</classname> class defines 
				an interface for using thread specific storage.
			</purpose>
			
			<description>
				<para>Thread specific storage is data associated with 
				individual threads and is often used to make operations
				that rely on global data 
				<link linkend="thread.glossary.thread-safe">thread-safe</link>.
				</para>
				
				<para>Template <classname>thread_specific_ptr</classname> 
				stores a pointer to an object obtained on a thread-by-thread
				basis and calls a specified cleanup handler on the contained
				pointer when the thread terminates. The cleanup handlers are
				called in the reverse order of construction of the 
				<classname>thread_specific_ptr</classname>s, and for the 
				initial thread are called by the destructor, providing the 
				same ordering guarantees as for normal declarations. Each
				thread initially stores the null pointer in each
				<classname>thread_specific_ptr</classname> instance.</para>
				
				<para>The template <classname>thread_specific_ptr</classname>
				is useful in the following cases:
					<itemizedlist>
						<listitem>An interface was originally written assuming 
						a single thread of control and it is being ported to a
						multithreaded environment.</listitem>
						
						<listitem>Each thread of control invokes sequences of
						methods that share data that are physically unique
						for each thread, but must be logically accessed
						through a globally visible access point instead of 
						being explicitly passed.</listitem>
					</itemizedlist>
				</para>
			</description>
			
			<inherit access="private">
				<type><classname>boost::noncopyable</classname></type>
				<purpose>Exposition only</purpose>
			</inherit>
			
			<constructor>
				<requires>The expression <code>delete get()</code> is well
				formed.</requires>
				
				<effects>A thread-specific data key is allocated and visible to
				all threads in the process. Upon creation, the value 
				<code>NULL</code> will be associated with the new key in all 
				active threads. A cleanup method is registered with the key 
				that will call <code>delete</code> on the value associated 
				with the key for a thread when it exits. When a thread exits,
				if a key has a registered cleanup method and the thread has a
				non-<code>NULL</code> value associated with that key, the value
				of the key is set to <code>NULL</code> and then the cleanup 
				method is called with the previously associated value as its 
				sole argument. The order in which registered cleanup methods 
				are called when a thread exits is undefined. If after all the
				cleanup methods have been called for all non-<code>NULL</code>
				values, there are still some non-<code>NULL</code> values
				with associated cleanup handlers the result is undefined
				behavior.</effects>
				
				<throws><classname>boost::thread_resource_error</classname> if
				the necessary resources can not be obtained.</throws>
				
				<notes>There may be an implementation specific limit to the 
				number of thread specific storage objects that can be created,
				and this limit may be small.</notes>
				
				<rationale>The most common need for cleanup will be to call 
				<code>delete</code> on the associated value. If other forms
				of cleanup are required the overloaded constructor should be
				called instead.</rationale>
			</constructor>
			
			<constructor>
				<parameter name="cleanup">
					<paramtype>void (*cleanup)(void*)</paramtype>
				</parameter>
				
				<effects>A thread-specific data key is allocated and visible to
				all threads in the process. Upon creation, the value 
				<code>NULL</code> will be associated with the new key in all 
				active threads. The <code>cleanup</code> method is registered
				with the key and will be called for a thread with the value 
				associated with the key for that thread when it exits. When a
				thread exits, if a key has a registered cleanup method and the
				thread has a non-<code>NULL</code> value associated with that
				key, the value of the key is set to <code>NULL</code> and then
				the cleanup method is called with the previously associated
				value as its sole argument. The order in which registered
				cleanup methods are called when a thread exits is undefined.
				If after all the cleanup methods have been called for all 
				non-<code>NULL</code> values, there are still some 
				non-<code>NULL</code> values with associated cleanup handlers
				the result is undefined behavior.</effects>
				
				<throws><classname>boost::thread_resource_error</classname> if
				the necessary resources can not be obtained.</throws>
				
				<notes>There may be an implementation specific limit to the 
				number of thread specific storage objects that can be created,
				 and this limit may be small.</notes>
				 
				 <rationale>There is the occasional need to register 
				 specialized cleanup methods, or to register no cleanup method
				 at all (done by passing <code>NULL</code> to this constructor.
				 </rationale>
			</constructor>
			
			<destructor>
				<effects>Deletes the thread-specific data key allocated by the
				constructor. The thread-specific data values associated with
				the key need not be <code>NULL</code>. It is the responsibility
				of the application to perform any cleanup actions for data
				associated with the key.</effects>
				
				<notes>Does not destroy any data that may be stored in any
				thread's thread specific storage. For this reason you should
				not destroy a <classname>thread_specific_ptr</classname> object
				until you are certain there are no threads running that have
				made use of its thread specific storage.</notes>
				
				<rationale>Associated data is not cleaned up because registered
				cleanup methods need to be run in the thread that allocated the
				associated data to be guarranteed to work correctly. There's no
				safe way to inject the call into another thread's execution
				path, making it impossible to call the cleanup methods safely.
				</rationale>
			</destructor>
			
			<method-group name="modifier functions">
				<method name="release">
					<type>T*</type>
					
					<postconditions><code>*this</code> holds the null pointer
					for the current thread.</postconditions>
					
					<returns><code>this-&gt;get()</code> prior to the call.</returns>
					
					<rationale>This method provides a mechanism for the user to
					relinquish control of the data associated with the 
					thread-specific key.</rationale>
				</method>
			
				<method name="reset">
					<type>void</type>

					<parameter name="p">
						<paramtype>T*</paramtype>
						<default>0</default>
					</parameter>
					
					<effects>If <code>this-&gt;get() != p &amp;&amp; 
					this-&gt;get() != NULL</code> then call the 
					associated cleanup function.</effects>
					
					<postconditions><code>*this</code> holds the pointer 
					<code>p</code> for the current thread.</postconditions>
				</method>
			</method-group>
			
			<method-group name="observer functions">
				<method name="get" cv="const">
					<type>T*</type>
					
					<returns>The object stored in thread specific storage for
					the current thread for <code>*this</code>.</returns>
					
					<notes>Each thread initially returns 0.</notes>
				</method>
				
				<method name="operator-&gt;" cv="const">
					<type>T*</type>
					
					<returns><code>this-&gt;get()</code>.</returns>
				</method>
				
				<method name="operator*()" cv="const">
					<type>T&amp;</type>
					
					<requires><code>this-&gt;get() != 0</code></requires>
					
					<returns><code>this-&gt;get()</code>.</returns>
				</method>
			</method-group>
		</class>
	</namespace>
</header>
