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Lock-free implementation documented, minor changes to align with TR1

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[SVN r29536]
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Peter Dimov
2005-06-12 12:58:17 +00:00
parent 24c23b8064
commit 675d09723a
2 changed files with 51 additions and 55 deletions
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98
shared_ptr.htm
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@ -20,7 +20,7 @@
<A href="sp_techniques.html">Programming Techniques</A></p>
<h2><a name="Introduction">Introduction</a></h2>
<p>The <b>shared_ptr</b> class template stores a pointer to a dynamically allocated
object, typically with a C++ <EM>new-expression</EM> . The object pointed to is
object, typically with a C++ <EM>new-expression</EM>. The object pointed to is
guaranteed to be deleted when the last <b>shared_ptr</b> pointing to it is
destroyed or reset. See the <A href="#example">example</A>.</p>
<p>Every <b>shared_ptr</b> meets the <b>CopyConstructible</b> and <b>Assignable</b>
@ -47,6 +47,13 @@
to <STRONG>shared_ptr&lt;T const&gt;</STRONG>, to <STRONG>shared_ptr&lt;U&gt;</STRONG>
where <STRONG>U</STRONG> is an accessible base of <STRONG>T</STRONG>, and to <STRONG>
shared_ptr&lt;void&gt;</STRONG>.</P>
<P><STRONG>shared_ptr</STRONG> is now part of <STRONG>TR1</STRONG>, the first C++
Library Technical Report. The latest draft of <STRONG>TR1</STRONG> is available
at the following location:</P>
<P><A href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1745.pdf">http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1745.pdf</A> (1.36Mb
PDF)</P>
<P>This implementation conforms to the TR1 specification, with the only exception
that it resides in namespace <code>boost</code> instead of <code>std::tr1</code>.</P>
<h2><a name="BestPractices">Best Practices</a></h2>
<P>A simple guideline that nearly eliminates the possibility of memory leaks is:
always use a named smart pointer variable to hold the result of <STRONG>new. </STRONG>
@ -154,15 +161,6 @@ void bad()
template&lt;class D, class T&gt;
D * <A href="#get_deleter">get_deleter</A>(shared_ptr&lt;T&gt; const &amp; p);
}</pre>
<P><EM>[It might be convenient to relax the requirements on <STRONG>shared_ptr</STRONG>'s
signature, allowing an additional, defaulted, template parameter; the parameter
can encode the threading model, for example. This would help in detecting
possible ODR violations.</EM></P>
<P><EM>On the other hand, using <STRONG>shared_ptr</STRONG> as an argument to a
template template parameter requires an exact signature match. </EM><EM>Metaprogramming
experts tend to deemphasize template template parameters as they are too
inflexible, but the alternative is typically an std::allocator::rebind-type
"hack".]</EM></P>
<h2><a name="Members">Members</a></h2>
<h3><a name="element_type">element_type</a></h3>
<pre>typedef T element_type;</pre>
@ -174,22 +172,12 @@ void bad()
<blockquote>
<p><b>Effects:</b> Constructs an <EM>empty</EM> <b>shared_ptr</b>. <EM>Empty</EM> <STRONG>
shared_ptr</STRONG> objects have an unspecified <A href="#use_count">use_count</A>.</p>
<p><b>Postconditions:</b> <code>get() == 0</code>.</p>
<p><b>Postconditions:</b> <code>use_count() == 0 &amp;&amp; get() == 0</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<P><EM>[The nothrow guarantee is important, since <STRONG>reset()</STRONG> is specified
in terms of the default constructor; this implies that the constructor must not
allocate memory.</EM></P>
<P><EM>There are two possible implementations, one stores 0 as a pointer to the
reference count, the other uses a single statically allocated count for all
default-constructed <STRONG>shared_ptr</STRONG>s. The second option is
difficult to achieve in the current header-only reference implementation due to
thread safety issues and initialization order, but it should not be precluded
by the specification. That's why the use_count() has been left unspecified.</EM></P>
<P><EM>A future release may enable <STRONG>shared_ptr</STRONG> construction from a
literal zero, for consistency with built-in pointers. It is not clear yet
whether this constructor should be left implicit, enabling <STRONG>0</STRONG> to
be used as a shorthand for <STRONG>shared_ptr&lt;T&gt;().</STRONG>]</EM></P>
allocate memory.]</EM></P>
<pre>template&lt;class Y&gt; explicit shared_ptr(Y * p);</pre>
<blockquote>
<p><b>Requirements:</b> <b>p</b> must be convertible to <b>T *</b>. <STRONG>Y</STRONG>
@ -198,8 +186,8 @@ void bad()
</p>
<p><b>Effects:</b> Constructs a <b>shared_ptr</b> that <EM>owns</EM> the pointer <b>p</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == 1 &amp;&amp; get() == p</code>.</p>
<p><b>Throws:</b> <b>std::bad_alloc</b> or an implementation-defined exception when
a resource other than memory could not be obtained.</p>
<p><b>Throws:</b> <STRONG>std::bad_alloc</STRONG>, or an implementation-defined
exception when a resource other than memory could not be obtained.</p>
<p><b>Exception safety:</b> If an exception is thrown, <code>delete p</code> is
called.</p>
<P><STRONG>Notes:</STRONG> <B>p</B> must be a pointer to an object that was
@ -226,8 +214,8 @@ void bad()
<p><b>Effects:</b> Constructs a <b>shared_ptr</b> that <EM>owns</EM> the pointer <STRONG>
p</STRONG> and the deleter <b>d</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == 1 &amp;&amp; get() == p</code>.</p>
<p><b>Throws:</b> <b>std::bad_alloc</b> or an implementation-defined exception when
a resource other than memory could not be obtained.</p>
<p><b>Throws:</b> <STRONG>std::bad_alloc</STRONG>, or an implementation-defined
exception when a resource other than memory could not be obtained.</p>
<p><b>Exception safety:</b> If an exception is thrown, <code>d(p)</code> is called.</p>
<p><b>Notes:</b> When the the time comes to delete the object pointed to by <b>p</b>,
the stored copy of <STRONG>d</STRONG> is invoked with the stored copy of <STRONG>p</STRONG>
@ -245,17 +233,10 @@ void bad()
<P><EM>The requirement that the copy constructor of <b>D</b> does not throw comes from
the pass by value. If the copy constructor throws, the pointer is leaked.
Removing the requirement requires a pass by (const) reference.</EM></P>
<P><EM>Pass by reference is problematic since (1) pass by value conveniently changes
functions (function references) to function pointers (this has to be performed
manually otherwise and some compilers may not be able to do it) and (2) const
references don't currently (per the standard) bind to functions. This can be
solved (I think) but it requires an overload set that breaks on many compilers
due to 14.5.5.2 problems (and of course it will break on compilers that don't
do partial ordering at all.)</EM></P>
<P><EM>The main problem with pass by reference, though, lies in its interaction with
rvalues. A const reference may still cause a copy, and will require a const
operator(). A non-const reference won't bind to an rvalue at all. A good
solution to this problem is the rvalue reference proposed in <A href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1377.htm">
<P><EM>The main problem with pass by reference lies in its interaction with rvalues. A
const reference may still cause a copy, and will require a const operator(). A
non-const reference won't bind to an rvalue at all. A good solution to this
problem is the rvalue reference proposed in <A href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1377.htm">
N1377</A>/<A href="http://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2002/n1385.htm">N1385</A>.]</EM></P>
<pre>shared_ptr(shared_ptr const &amp; r); // never throws
template&lt;class Y&gt; shared_ptr(shared_ptr&lt;Y&gt; const &amp; r); // never throws</pre>
@ -280,8 +261,8 @@ template&lt;class Y&gt; shared_ptr(shared_ptr&lt;Y&gt; const &amp; r); // never
<BLOCKQUOTE>
<P><B>Effects:</B> Constructs a <B>shared_ptr</B>, as if by storing a copy of <STRONG>r.release()</STRONG>.</P>
<p><b>Postconditions:</b> <code>use_count() == 1</code>.</p>
<p><b>Throws:</b> <b>std::bad_alloc</b> or an implementation-defined exception when
a resource other than memory could not be obtained.</p>
<p><b>Throws:</b> <STRONG>std::bad_alloc</STRONG>, or an implementation-defined
exception when a resource other than memory could not be obtained.</p>
<P><B>Exception safety:</B> If an exception is thrown, the constructor has no
effect.</P>
</BLOCKQUOTE>
@ -325,12 +306,6 @@ q = p;
</pre>
<p>both assignments may be no-ops.</p>
</BLOCKQUOTE>
<P><EM>[Some experts consider the note to be redundant, as it appears to essentially
mirror the "as if" rule. However, experience suggests that when C++ code is
used to describe effects, it is often misinterpreted as required
implementation. In addition, it is not entirely clear whether the "as if" rule
actually applies here, so it's better to be explicit about the possible
optimizations.]</EM></P>
<h3><a name="reset">reset</a></h3>
<pre>void reset(); // never throws</pre>
<BLOCKQUOTE>
@ -602,11 +577,32 @@ p3.reset(new int(1));
// thread B
p3.reset(new int(2)); // undefined, multiple writes
</pre>
<p><STRONG>shared_ptr</STRONG> uses <A href="../config/config.htm">Boost.Config</A>
to detect whether the implementation supports threads. If your program is
single-threaded, but your platform is autodetected by <STRONG>Boost.Config</STRONG>
as supporting multiple threads, <STRONG>#define BOOST_DISABLE_THREADS</STRONG> to
eliminate the thread safety overhead.</p>
<p>&nbsp;</p>
<P>Starting with Boost release 1.33.0, <STRONG>shared_ptr</STRONG> uses a lock-free
implementation on the following platforms:</P>
<UL>
<LI>
GNU GCC on x86 or x86-64;</LI>
<LI>
GNU GCC on IA64;</LI>
<LI>
Metrowerks CodeWarrior on PowerPC;</LI>
<LI>
GNU GCC on PowerPC;</LI>
<LI>
Windows.</LI></UL>
<P>If your program is single-threaded and does not link to any libraries that might
have used <STRONG>shared_ptr</STRONG> in its default configuration, you can <STRONG>
#define</STRONG> the macro <STRONG>BOOST_SP_DISABLE_THREADS</STRONG> on a
project-wide basis to switch to ordinary non-atomic reference count updates.</P>
<P>(Defining <STRONG>BOOST_SP_DISABLE_THREADS</STRONG> in some, but not
all, translation units is technically a violation of the One Definition
Rule and undefined behavior. Nevertheless, the implementation attempts to do
its best to accommodate the request to use non-atomic updates in those
translation units. No guarantees, though.)</P>
<P>You can define the macro <STRONG>BOOST_SP_USE_PTHREADS</STRONG> to turn off the
lock-free platform-specific implementation and fall back to the generic <STRONG>
pthread_mutex_t</STRONG>-based code.</P>
<h2><a name="FAQ">Frequently Asked Questions</a></h2>
<P><B>Q.</B> There are several variations of shared pointers, with different
tradeoffs; why does the smart pointer library supply only a single
@ -691,7 +687,7 @@ int * p = a.release();
<p>
$Date$</p>
<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Copyright 2002, 2003 Peter Dimov. Permission to copy, use, modify, sell and
Copyright 2002-2005 Peter Dimov. Permission to copy, use, modify, sell and
distribute this document is granted provided this copyright notice appears in
all copies. This document is provided "as is" without express or implied
warranty, and with no claim as to its suitability for any purpose.</small></p>

8
weak_ptr.htm
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@ -145,9 +145,9 @@ template&lt;class Y&gt; weak_ptr &amp; operator=(shared_ptr&lt;Y&gt; const &amp;
<h3><a name="use_count">use_count</a></h3>
<pre>long use_count() const;</pre>
<blockquote>
<p><b>Returns:</b> if <STRONG>*this</STRONG> is <EM>empty</EM>, an unspecified
nonnegative value; otherwise, the number of <b>shared_ptr</b> objects that <EM>share
ownership</EM> with <STRONG>*this</STRONG>.</p>
<p><b>Returns:</b> 0 if <STRONG>*this</STRONG> is <EM>empty</EM>; otherwise,
the number of <b>shared_ptr</b> objects that <EM>share ownership</EM>
with <STRONG>*this</STRONG>.</p>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> <code>use_count()</code> is not necessarily efficient. Use only
for debugging and testing purposes, not for production code.</P>
@ -236,7 +236,7 @@ public:
<hr>
<p>$Date$</p>
<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Copyright 2002, 2003 Peter Dimov. Permission to copy, use, modify, sell and
Copyright 2002-2005 Peter Dimov. Permission to copy, use, modify, sell and
distribute this document is granted provided this copyright notice appears in
all copies. This document is provided "as is" without express or implied
warranty, and with no claim as to its suitability for any purpose.</small></p>