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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Smart Pointers</title>
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<h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
width="277" align="middle" border="0">Smart Pointers</h1>
<p><a href="#Introduction">Introduction</a><br>
<a href="#common_requirements">Common Requirements</a><br>
<a href="#Exception_Safety">Exception Safety</a><br>
<a href="#Exception-specifications">Exception-specifications</a><br>
<a href="#History">History and Acknowledgements</a><br>
<a href="#References">References</a></p>
<h2><a name="Introduction">Introduction</a></h2>
<p>Smart pointers are objects which store pointers to dynamically allocated (heap)
objects. They behave much like built-in C++ pointers except that they
automatically delete the object pointed to at the appropriate time. Smart
pointers are particularly useful in the face of exceptions as they ensure
proper destruction of dynamically allocated objects. They can also be used to
keep track of dynamically allocated objects shared by multiple owners.</p>
<p>Conceptually, smart pointers are seen as owning the object pointed to, and thus
responsible for deletion of the object when it is no longer needed.</p>
<p>The smart pointer library provides six smart pointer class templates:</p>
<div align="left">
<table border="1" cellpadding="4" cellspacing="0">
<tr>
<td><a href="scoped_ptr.htm"><b>scoped_ptr</b></a></td>
<td><a href="../../boost/scoped_ptr.hpp"><boost/scoped_ptr.hpp></a></td>
<td>Simple sole ownership of single objects. Noncopyable.</td>
</tr>
<tr>
<td><a href="scoped_array.htm"><b>scoped_array</b></a></td>
<td><a href="../../boost/scoped_array.hpp"><boost/scoped_array.hpp></a></td>
<td>Simple sole ownership of arrays. Noncopyable.</td>
</tr>
<tr>
<td><a href="shared_ptr.htm"><b>shared_ptr</b></a></td>
<td><a href="../../boost/shared_ptr.hpp"><boost/shared_ptr.hpp></a></td>
<td>Object ownership shared among multiple pointers.</td>
</tr>
<tr>
<td><a href="shared_array.htm"><b>shared_array</b></a></td>
<td><a href="../../boost/shared_array.hpp"><boost/shared_array.hpp></a></td>
<td>Array ownership shared among multiple pointers.</td>
</tr>
<tr>
<td><a href="weak_ptr.htm"><b>weak_ptr</b></a></td>
<td><a href="../../boost/weak_ptr.hpp"><boost/weak_ptr.hpp></a></td>
<td>Non-owning observers of an object owned by <b>shared_ptr</b>.</td>
</tr>
<tr>
<td><a href="intrusive_ptr.html"><b>intrusive_ptr</b></a></td>
<td><a href="../../boost/intrusive_ptr.hpp"><boost/intrusive_ptr.hpp></a></td>
<td>Shared ownership of objects with an embedded reference count.</td>
</tr>
</table>
</div>
<p>These templates are designed to complement the <b>std::auto_ptr</b> template.</p>
<p>They are examples of the "resource acquisition is initialization" idiom
described in Bjarne Stroustrup's "The C++ Programming Language", 3rd edition,
Section 14.4, Resource Management.</p>
<p>Additionally, the smart pointer library provides efficient factory functions
for creating smart pointer objects:</p>
<div align="left">
<table border="1" cellpadding="4" cellspacing="0">
<tr>
<td><a href="make_shared.html"><b>make_shared, allocate_shared</b></a> for objects</td>
<td><a href="../../boost/make_shared.hpp"><boost/make_shared.hpp></a></td>
<td>Efficient creation of <code>shared_ptr</code> objects.</td>
</tr>
<tr>
<td><a href="make_shared_array.html"><b>make_shared, allocate_shared</b></a> for arrays</td>
<td><a href="../../boost/make_shared.hpp"><boost/make_shared.hpp></a></td>
<td>Efficient creation of <code>shared_ptr</code> arrays.</td>
</tr>
<tr>
<td><a href="make_unique.html"><b>make_unique</b></a></td>
<td><a href="../../boost/make_unique.hpp"><boost/make_unique.hpp></a></td>
<td>Creation of <code>unique_ptr</code> objects and arrays.</td>
</tr>
</table>
</div>
<p>A test program, <a href="test/smart_ptr_test.cpp">smart_ptr_test.cpp</a>, is
provided to verify correct operation.</p>
<p>A page on <a href="compatibility.htm">compatibility</a> with older versions of
the Boost smart pointer library describes some of the changes since earlier
versions of the smart pointer implementation.</p>
<p>A page on <a href="smarttests.htm">smart pointer timings</a> will be of interest
to those curious about performance issues.</p>
<P>A page on <A href="sp_techniques.html">smart pointer programming techniques</A> lists
some advanced applications of <code>shared_ptr</code> and <code>weak_ptr</code>.</P>
<h2><a name="common_requirements">Common Requirements</a></h2>
<p>These smart pointer class templates have a template parameter, <b>T</b>, which
specifies the type of the object pointed to by the smart pointer. The behavior
of the smart pointer templates is undefined if the destructor or <b>operator delete</b>
for objects of type <b>T</b> throw exceptions.</p>
<p><b>T</b> may be an incomplete type at the point of smart pointer declaration.
Unless otherwise specified, it is required that <b>T</b> be a complete type at
points of smart pointer instantiation. Implementations are required to diagnose
(treat as an error) all violations of this requirement, including deletion of
an incomplete type. See the description of the <a href="../utility/utility.htm#checked_delete">
<b>checked_delete</b></a> function template.</p>
<P>Note that <STRONG>shared_ptr</STRONG> does not have this restriction, as most of
its member functions do not require <STRONG>T</STRONG> to be a complete type.</P>
<h3>Rationale</h3>
<p>The requirements on <b>T</b> are carefully crafted to maximize safety yet allow
handle-body (also called pimpl) and similar idioms. In these idioms a smart
pointer may appear in translation units where <b>T</b> is an incomplete type.
This separates interface from implementation and hides implementation from
translation units which merely use the interface. Examples described in the
documentation for specific smart pointers illustrate use of smart pointers in
these idioms.</p>
<p>Note that <b>scoped_ptr</b> requires that <b>T</b> be a complete type at
destruction time, but <b>shared_ptr</b> does not.</p>
<h2><a name="Exception_Safety">Exception Safety</a></h2>
<p>Several functions in these smart pointer classes are specified as having "no
effect" or "no effect except such-and-such" if an exception is thrown. This
means that when an exception is thrown by an object of one of these classes,
the entire program state remains the same as it was prior to the function call
which resulted in the exception being thrown. This amounts to a guarantee that
there are no detectable side effects. Other functions never throw exceptions.
The only exception ever thrown by functions which do throw (assuming <b>T</b> meets
the <a href="#common_requirements">common requirements</a>) is <b>std::bad_alloc</b>,
and that is thrown only by functions which are explicitly documented as
possibly throwing <b>std::bad_alloc</b>.</p>
<h2><a name="Exception-specifications">Exception-specifications</a></h2>
<p>Exception-specifications are not used; see <a href="http://www.boost.org/more/lib_guide.htm#Exception-specification">
exception-specification rationale</a>.</p>
<p>All the smart pointer templates contain member functions which can never throw
exceptions, because they neither throw exceptions themselves nor call other
functions which may throw exceptions. These members are indicated by a comment: <code>
// never throws</code>.
</p>
<p>Functions which destroy objects of the pointed to type are prohibited from
throwing exceptions by the <a href="#common_requirements">common requirements</a>.</p>
<h2><a name="History">History</a> and Acknowledgements</h2>
<p>February 2014. Glen Fernandes updated overloads of <b>make_shared</b> and
<b>allocate_shared</b> to conform to the specification in C++ standard paper
<a href="#D&F-14">[D&F-14]</a>, and implemented <b>make_unique</b> for
arrays and objects. Peter Dimov and Glen Fernandes updated the scalar and
array implementations, respectively, to resolve C++ standard library defect
2070.</p>
<p>November 2012. Glen Fernandes provided implementations of <b>make_shared</b>
and <b>allocate_shared</b> for arrays. They achieve a single allocation for an
array that can be initialized with constructor arguments or initializer lists
as well as overloads for default initialization and no value initialization.
See the <a href="make_shared_array.html">make_shared and allocate_shared for
arrays</a> page for more information.</p>
<p>January 2002. Peter Dimov reworked all four classes, adding features, fixing
bugs, and splitting them into four separate headers, and added <b>weak_ptr</b>.
See the <a href="compatibility.htm">compatibility</a> page for a summary of the
changes.</p>
<p>May 2001. Vladimir Prus suggested requiring a complete type on destruction.
Refinement evolved in discussions including Dave Abrahams, Greg Colvin, Beman
Dawes, Rainer Deyke, Peter Dimov, John Maddock, Vladimir Prus, Shankar Sai, and
others.</p>
<p>November 1999. Darin Adler provided <b>operator ==</b>, <b>operator !=</b>, and <b>std::swap</b>
and <b>std::less</b> specializations for shared types.</p>
<p>September 1999. Luis Coelho provided <b>shared_ptr::swap</b> and <b>shared_array::swap</b></p>
<p>May 1999. In April and May, 1999, Valentin Bonnard and David Abrahams made a
number of suggestions resulting in numerous improvements.</p>
<p>October 1998. Beman Dawes proposed reviving the original semantics under the
names <b>safe_ptr</b> and <b>counted_ptr</b>, meeting of Per Andersson, Matt
Austern, Greg Colvin, Sean Corfield, Pete Becker, Nico Josuttis, Dietmar Kühl,
Nathan Myers, Chichiang Wan and Judy Ward. During the discussion, the four new
class names were finalized, it was decided that there was no need to exactly
follow the <b>std::auto_ptr</b> interface, and various function signatures and
semantics were finalized.</p>
<p>Over the next three months, several implementations were considered for <b>shared_ptr</b>,
and discussed on the <a href="http://www.boost.org">boost.org</a> mailing list.
The implementation questions revolved around the reference count which must be
kept, either attached to the pointed to object, or detached elsewhere. Each of
those variants have themselves two major variants:
<ul>
<li>
Direct detached: the shared_ptr contains a pointer to the object, and a pointer
to the count.
<li>
Indirect detached: the shared_ptr contains a pointer to a helper object, which
in turn contains a pointer to the object and the count.
<li>
Embedded attached: the count is a member of the object pointed to.
<li>
Placement attached: the count is attached via operator new manipulations.</li>
</ul>
<p>Each implementation technique has advantages and disadvantages. We went so far
as to run various timings of the direct and indirect approaches, and found that
at least on Intel Pentium chips there was very little measurable difference.
Kevlin Henney provided a paper he wrote on "Counted Body Techniques." Dietmar
Kühl suggested an elegant partial template specialization technique to allow
users to choose which implementation they preferred, and that was also
experimented with.</p>
<p>But Greg Colvin and Jerry Schwarz argued that "parameterization will discourage
users", and in the end we choose to supply only the direct implementation.</p>
<p>Summer, 1994. Greg Colvin proposed to the C++ Standards Committee classes named <b>auto_ptr</b>
and <b>counted_ptr</b> which were very similar to what we now call <b>scoped_ptr</b>
and <b>shared_ptr</b>. <a href="#Col-94">[Col-94]</a> In one of the very few
cases where the Library Working Group's recommendations were not followed by
the full committee, <b>counted_ptr</b> was rejected and surprising
transfer-of-ownership semantics were added to <b>auto_ptr</b>.</p>
<h2><a name="References">References</a></h2>
<p>[<a name="D&F-14">D&F-14</a>] Peter Dimov & Glen Fernandes, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3870.html">
Extending make_shared to Support Arrays, Revision 1</a>, C++ committee document N3870,
January, 2014.</p>
<p>[<a name="Col-94">Col-94</a>] Gregory Colvin, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/1994/N0555.pdf">
Exception Safe Smart Pointers</a>, C++ committee document 94-168/N0555,
July, 1994.</p>
<p>[<a name="E&D-94">E&D-94</a>] John R. Ellis & David L. Detlefs, <a href="http://www.usenix.org/publications/library/proceedings/c++94/full_papers/ellis.a">
Safe, Efficient Garbage Collection for C++</a>, Usenix Proceedings,
February, 1994. This paper includes an extensive discussion of weak pointers
and an extensive bibliography.</p>
<hr>
<p>$Date$</p>
<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Distributed under the Boost Software License, Version 1.0. See accompanying
file <A href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or copy at
<A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>.</small></p>
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