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<h1><A href="../../index.htm"><IMG height="86" alt="boost.png (6897 bytes)" src="../../boost.png" width="277" align="middle"
border="0"></A>shared_ptr class template</h1>
<p><A href="#Introduction">Introduction</A><br>
<A href="#BestPractices">Best Practices</A><br>
<A href="#Synopsis">Synopsis</A><br>
<A href="#Members">Members</A><br>
<A href="#functions">Free Functions</A><br>
<A href="#example">Example</A><br>
<A href="#Handle/Body">Handle/Body Idiom</A><br>
<A href="#ThreadSafety">Thread Safety</A><br>
<A href="#FAQ">Frequently Asked Questions</A><br>
<A href="smarttests.htm">Smart Pointer Timings</A><br>
<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
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>
requirements of the C++ Standard Library, and so can be used in standard
library containers. Comparison operators are supplied so that <b>shared_ptr</b>
works with the standard library's associative containers.</p>
<p>Normally, a <b>shared_ptr</b> cannot correctly hold a pointer to a dynamically
allocated array. See <A href="shared_array.htm"><b>shared_array</b></A> for
that usage.</p>
<p>Because the implementation uses reference counting, cycles of <b>shared_ptr</b> instances
will not be reclaimed. For example, if <b>main()</b> holds a <b>shared_ptr</b> to
<b>A</b>, which directly or indirectly holds a <b>shared_ptr</b> back to <b>A</b>,
<b>A</b>'s use count will be 2. Destruction of the original <b>shared_ptr</b> will
leave <b>A</b> dangling with a use count of 1. Use <A href="weak_ptr.htm">weak_ptr</A>
to "break cycles."</p>
<p>The class template is parameterized on <b>T</b>, the type of the object pointed
to. <STRONG>shared_ptr</STRONG> and most of its member functions place no
requirements on <STRONG>T</STRONG>; it is allowed to be an incomplete type, or <STRONG>
void</STRONG>. Member functions that do place additional requirements (<A href="#constructors">constructors</A>,
<A href="#reset">reset</A>) are explicitly documented below.</p>
<P><STRONG>shared_ptr<T></STRONG> can be implicitly converted to <STRONG>shared_ptr<U></STRONG>
whenever <STRONG>T*</STRONG> can be implicitly converted to <STRONG>U*</STRONG>.
In particular, <STRONG>shared_ptr<T></STRONG> is implicitly convertible
to <STRONG>shared_ptr<T const></STRONG>, to <STRONG>shared_ptr<U></STRONG>
where <STRONG>U</STRONG> is an accessible base of <STRONG>T</STRONG>, and to <STRONG>
shared_ptr<void></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>
Every occurence of the <STRONG>new</STRONG> keyword in the code should have the
form:</P>
<PRE>shared_ptr<T> p(new Y);</PRE>
<P>It is, of course, acceptable to use another smart pointer in place of <STRONG>shared_ptr</STRONG>
above; having <STRONG>T</STRONG> and <STRONG>Y</STRONG> be the same type, or
passing arguments to <STRONG>Y</STRONG>'s constructor is also OK.</P>
<P>If you observe this guideline, it naturally follows that you will have no
explicit <STRONG>delete</STRONG>s; <STRONG>try/catch</STRONG> constructs will
be rare.</P>
<P>Avoid using unnamed <STRONG>shared_ptr</STRONG> temporaries to save typing; to
see why this is dangerous, consider this example:</P>
<PRE>void f(shared_ptr<int>, int);
int g();
void ok()
{
shared_ptr<int> p(new int(2));
f(p, g());
}
void bad()
{
f(shared_ptr<int>(new int(2)), g());
}
</PRE>
<P>The function <STRONG>ok</STRONG> follows the guideline to the letter, whereas <STRONG>
bad</STRONG> constructs the temporary <STRONG>shared_ptr</STRONG> in place,
admitting the possibility of a memory leak. Since function arguments are
evaluated in unspecified order, it is possible for <STRONG>new int(2)</STRONG> to
be evaluated first, <STRONG>g()</STRONG> second, and we may never get to the <STRONG>
shared_ptr </STRONG>constructor if <STRONG>g</STRONG> throws an exception.
See <A href="http://www.gotw.ca/gotw/056.htm">Herb Sutter's treatment</A> (also <A href="http://www.cuj.com/reference/articles/2002/0212/0212_sutter.htm">
here</A>) of the issue for more information.</P>
<P>The exception safety problem described above may also be eliminated by using
the <a href="make_shared.html"><code>make_shared</code></a>
or <a href="make_shared.html"><code>allocate_shared</code></a>
factory functions defined in boost/make_shared.hpp. These factory functions also provide
an efficiency benefit by consolidating allocations.<P>
<h2><a name="Synopsis">Synopsis</a></h2>
<pre>namespace boost {
class bad_weak_ptr: public std::exception;
template<class T> class <A href="weak_ptr.htm" >weak_ptr</A>;
template<class T> class shared_ptr {
public:
typedef T <A href="#element_type" >element_type</A>;
<A href="#constructors" >shared_ptr</A>(); // never throws
template<class Y> explicit <A href="#constructors" >shared_ptr</A>(Y * p);
template<class Y, class D> <A href="#constructors" >shared_ptr</A>(Y * p, D d);
template<class Y, class D, class A> <A href="#allocator_constructor" >shared_ptr</A>(Y * p, D d, A a);
<A href="#destructor" >~shared_ptr</A>(); // never throws
<A href="#constructors" >shared_ptr</A>(shared_ptr const & r); // never throws
template<class Y> <A href="#constructors" >shared_ptr</A>(shared_ptr<Y> const & r); // never throws
template<class Y> <A href="#constructors" >shared_ptr</A>(shared_ptr<Y> const & r, T * p); // never throws
template<class Y> explicit <A href="#constructors" >shared_ptr</A>(<A href="weak_ptr.htm" >weak_ptr</A><Y> const & r);
template<class Y> explicit <A href="#constructors" >shared_ptr</A>(std::auto_ptr<Y> & r);
shared_ptr & <A href="#assignment" >operator=</A>(shared_ptr const & r); // never throws
template<class Y> shared_ptr & <A href="#assignment" >operator=</A>(shared_ptr<Y> const & r); // never throws
template<class Y> shared_ptr & <A href="#assignment" >operator=</A>(std::auto_ptr<Y> & r);
void <A href="#reset" >reset</A>(); // never throws
template<class Y> void <A href="#reset" >reset</A>(Y * p);
template<class Y, class D> void <A href="#reset" >reset</A>(Y * p, D d);
template<class Y, class D, class A> void <A href="#reset" >reset</A>(Y * p, D d, A a);
template<class Y> void <A href="#reset" >reset</A>(shared_ptr<Y> const & r, T * p); // never throws
T & <A href="#indirection" >operator*</A>() const; // never throws
T * <A href="#indirection" >operator-></A>() const; // never throws
T * <A href="#get" >get</A>() const; // never throws
bool <A href="#unique" >unique</A>() const; // never throws
long <A href="#use_count" >use_count</A>() const; // never throws
operator <A href="#conversions" ><i>unspecified-bool-type</i></A>() const; // never throws
void <A href="#swap" >swap</A>(shared_ptr & b); // never throws
};
template<class T, class U>
bool <A href="#comparison" >operator==</A>(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
template<class T, class U>
bool <A href="#comparison" >operator!=</A>(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
template<class T, class U>
bool <A href="#comparison" >operator<</A>(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws
template<class T> void <A href="#free-swap" >swap</A>(shared_ptr<T> & a, shared_ptr<T> & b); // never throws
template<class T> T * <A href="#get_pointer" >get_pointer</A>(shared_ptr<T> const & p); // never throws
template<class T, class U>
shared_ptr<T> <A href="#static_pointer_cast" >static_pointer_cast</A>(shared_ptr<U> const & r); // never throws
template<class T, class U>
shared_ptr<T> <A href="#const_pointer_cast" >const_pointer_cast</A>(shared_ptr<U> const & r); // never throws
template<class T, class U>
shared_ptr<T> <A href="#dynamic_pointer_cast" >dynamic_pointer_cast</A>(shared_ptr<U> const & r); // never throws
template<class E, class T, class Y>
std::basic_ostream<E, T> & <A href="#insertion-operator" >operator<<</A> (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);
template<class D, class T>
D * <A href="#get_deleter">get_deleter</A>(shared_ptr<T> const & p);
}</pre>
<h2><a name="Members">Members</a></h2>
<h3><a name="element_type">element_type</a></h3>
<pre>typedef T element_type;</pre>
<blockquote>
<p>Provides the type of the template parameter T.</p>
</blockquote>
<h3><a name="constructors">constructors</a></h3>
<pre>shared_ptr(); // never throws</pre>
<blockquote>
<p><b>Effects:</b> Constructs an <EM>empty</EM> <b>shared_ptr</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == 0 && 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>
<pre>template<class Y> explicit shared_ptr(Y * p);</pre>
<blockquote>
<p><b>Requirements:</b> <b>p</b> must be convertible to <b>T *</b>. <STRONG>Y</STRONG>
must be a complete type. The expression <code>delete p</code> must be
well-formed, must not invoke undefined behavior, and must not throw exceptions.
</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 && get() == p</code>.</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
allocated via a C++ <B>new</B> expression or be 0. The postcondition that <A href="#use_count">
use count</A> is 1 holds even if <b>p</b> is 0; invoking <STRONG>delete</STRONG>
on a pointer that has a value of 0 is harmless.</P>
</blockquote>
<P><EM>[This constructor has been changed to a template in order to remember the actual
pointer type passed. The destructor will call <STRONG>delete</STRONG> with the
same pointer, complete with its original type, even when <STRONG>T</STRONG> does
not have a virtual destructor, or is <STRONG>void</STRONG>.</EM></P>
<P><EM>The optional intrusive counting support has been dropped as it exposes too much
implementation details and doesn't interact well with <STRONG>weak_ptr</STRONG>.
The current implementation uses a different mechanism, <A href="enable_shared_from_this.html">
enable_shared_from_this</A>, to solve the "<STRONG>shared_ptr</STRONG> from <STRONG>
this</STRONG>" problem.</EM><EM>]</EM></P>
<a name="allocator_constructor"></a>
<pre>template<class Y, class D> shared_ptr(Y * p, D d);
template<class Y, class D, class A> shared_ptr(Y * p, D d, A a);</pre>
<blockquote>
<p><b>Requirements:</b> <B>p</B> must be convertible to <B>T *</B>. <STRONG>D</STRONG>
must be <STRONG>CopyConstructible</STRONG>. The copy constructor and destructor
of <b>D</b> must not throw. The expression <code>d(p)</code> must be
well-formed, must not invoke undefined behavior, and must not throw exceptions. <STRONG>
A</STRONG> must be an <EM>Allocator</EM>, as described in section 20.1.5 (<STRONG>Allocator
requirements</STRONG>) of the C++ Standard.
</p>
<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>. The second constructor allocates
memory using a copy of <STRONG>a</STRONG>.</p>
<p><b>Postconditions:</b> <code>use_count() == 1 && get() == p</code>.</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>
as an argument.</p>
</blockquote>
<P><EM>[Custom deallocators allow a factory function returning a <STRONG>shared_ptr</STRONG>
to insulate the user from its memory allocation strategy. Since the deallocator
is not part of the type, changing the allocation strategy does not break source
or binary compatibility, and does not require a client recompilation. For
example, a "no-op" deallocator is useful when returning a <STRONG>shared_ptr</STRONG>
to a statically allocated object, and other variations allow a <STRONG>shared_ptr</STRONG>
to be used as a wrapper for another smart pointer, easing interoperability.</EM></P>
<P><EM>The support for custom deallocators does not impose significant overhead. Other <STRONG>
shared_ptr</STRONG> features still require a deallocator to be kept.</EM></P>
<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>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 & r); // never throws
template<class Y> shared_ptr(shared_ptr<Y> const & r); // never throws</pre>
<blockquote>
<p><b>Effects:</b> If <b>r</b> is <EM>empty</EM>, constructs an <EM>empty</EM> <b>shared_ptr</b>;
otherwise, constructs a <b>shared_ptr</b> that <EM>shares ownership</EM> with <b>r</b>.</p>
<p><b>Postconditions:</b> <code>get() == r.get() && use_count() ==
r.use_count()</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<pre>template<class Y> shared_ptr(shared_ptr<Y> const & r, T * p); // never throws</pre>
<blockquote>
<p><b>Effects:</b> constructs a <b>shared_ptr</b> that <EM>shares ownership</EM> with
<b>r</b> and stores <b>p</b>.</p>
<p><b>Postconditions:</b> <code>get() == p && use_count() == r.use_count()</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<pre>template<class Y> explicit shared_ptr(<A href="weak_ptr.htm" >weak_ptr</A><Y> const & r);</pre>
<blockquote>
<p><b>Effects:</b> Constructs a <b>shared_ptr</b> that <EM>shares ownership</EM> with
<b>r</b> and stores a copy of the pointer stored in <STRONG>r</STRONG>.</p>
<p><b>Postconditions:</b> <code>use_count() == r.use_count()</code>.</p>
<p><b>Throws:</b> <b>bad_weak_ptr</b> when <code>r.use_count() == 0</code>.</p>
<p><b>Exception safety:</b> If an exception is thrown, the constructor has no
effect.</p>
</blockquote>
<pre>template<class Y> shared_ptr(std::auto_ptr<Y> & r);</pre>
<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> <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>
<P><EM>[This constructor takes a the source <STRONG>auto_ptr</STRONG> by reference and
not by value, and cannot accept <STRONG>auto_ptr</STRONG> temporaries. This is
by design, as the constructor offers the strong guarantee; an rvalue reference
would solve this problem, too.]</EM></P>
<h3><a name="destructor">destructor</a></h3>
<pre>~shared_ptr(); // never throws</pre>
<BLOCKQUOTE>
<P><B>Effects:</B></P>
<UL>
<LI>
If <STRONG>*this</STRONG> is <EM>empty</EM>, or <EM>shares ownership</EM> with
another <STRONG>shared_ptr</STRONG> instance (<code>use_count() > 1</code>),
there are no side effects.
<LI>
Otherwise, if <STRONG>*this</STRONG> <EM>owns</EM> a pointer <STRONG>p</STRONG>
and a deleter <STRONG>d</STRONG>, <code>d(p)</code>
is called.
<LI>
Otherwise, <STRONG>*this</STRONG> <EM>owns</EM> a pointer <STRONG>p</STRONG>,
and <code>delete p</code> is called.</LI></UL>
<P><B>Throws:</B> nothing.</P>
</BLOCKQUOTE>
<H3><a name="assignment">assignment</a></H3>
<pre>shared_ptr & operator=(shared_ptr const & r); // never throws
template<class Y> shared_ptr & operator=(shared_ptr<Y> const & r); // never throws
template<class Y> shared_ptr & operator=(std::auto_ptr<Y> & r);</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>shared_ptr(r).swap(*this)</code>.</P>
<P><B>Returns:</B> <code>*this</code>.</P>
<P><B>Notes:</B> The use count updates caused by the temporary object construction
and destruction are not considered observable side effects, and the
implementation is free to meet the effects (and the implied guarantees) via
different means, without creating a temporary. In particular, in the example:</P>
<pre>shared_ptr<int> p(new int);
shared_ptr<void> q(p);
p = p;
q = p;
</pre>
<p>both assignments may be no-ops.</p>
</BLOCKQUOTE>
<h3><a name="reset">reset</a></h3>
<pre>void reset(); // never throws</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>shared_ptr().swap(*this)</code>.</P>
</BLOCKQUOTE>
<pre>template<class Y> void reset(Y * p);</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>shared_ptr(p).swap(*this)</code>.</P>
</BLOCKQUOTE>
<pre>template<class Y, class D> void reset(Y * p, D d);</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>shared_ptr(p, d).swap(*this)</code>.</P>
</BLOCKQUOTE>
<pre>template<class Y, class D, class A> void reset(Y * p, D d, A a);</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>shared_ptr(p, d, a).swap(*this)</code>.</P>
</BLOCKQUOTE>
<pre>template<class Y> void reset(shared_ptr<Y> const & r, T * p); // never throws</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>shared_ptr(r, p).swap(*this)</code>.</P>
</BLOCKQUOTE>
<h3><a name="indirection">indirection</a></h3>
<pre>T & operator*() const; // never throws</pre>
<blockquote>
<p><b>Requirements:</b> The stored pointer must not be 0.</p>
<p><b>Returns:</b> a reference to the object pointed to by the stored pointer.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<pre>T * operator->() const; // never throws</pre>
<blockquote>
<p><b>Requirements:</b> The stored pointer must not be 0.</p>
<p><b>Returns:</b> the stored pointer.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<h3><a name="get">get</a></h3>
<pre>T * get() const; // never throws</pre>
<blockquote>
<p><b>Returns:</b> the stored pointer.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<h3><a name="unique">unique</a></h3>
<pre>bool unique() const; // never throws</pre>
<blockquote>
<p><b>Returns:</b> <code>use_count() == 1</code>.</p>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> <code>unique()</code> may be faster than <code>use_count()</code>.
If you are using <code>unique()</code> to implement copy on write, do not rely
on a specific value when the stored pointer is zero.</P>
</blockquote>
<h3><a name="use_count">use_count</a></h3>
<pre>long use_count() const; // never throws</pre>
<blockquote>
<p><b>Returns:</b> the number of <b>shared_ptr</b> objects, <STRONG>*this</STRONG> included,
that <i>share ownership</i> with <b>*this</b>, or 0 when <STRONG>*this</STRONG>
is <EM>empty</EM>.</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>
</blockquote>
<h3><a name="conversions">conversions</a></h3>
<pre>operator <i>unspecified-bool-type</i> () const; // never throws</pre>
<blockquote>
<p><b>Returns:</b> an unspecified value that, when used in boolean contexts, is
equivalent to <code>get() != 0</code>.</p>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> This conversion operator allows <b>shared_ptr</b> objects to be
used in boolean contexts, like <code>if (p && p->valid()) {}</code>.
The actual target type is typically a pointer to a member function, avoiding
many of the implicit conversion pitfalls.</P>
</blockquote>
<P><EM>[The conversion to bool is not merely syntactic sugar. It allows <STRONG>shared_ptr</STRONG>s
to be declared in conditions when using <A href="#dynamic_pointer_cast">dynamic_pointer_cast</A>
or <A href="weak_ptr.htm#lock">weak_ptr::lock</A>.]</EM></P>
<h3><a name="swap">swap</a></h3>
<pre>void swap(shared_ptr & b); // never throws</pre>
<blockquote>
<p><b>Effects:</b> Exchanges the contents of the two smart pointers.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<h2><a name="functions">Free Functions</a></h2>
<h3><a name="comparison">comparison</a></h3>
<pre>template<class T, class U>
bool operator==(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws</pre>
<blockquote>
<p><b>Returns:</b> <code>a.get() == b.get()</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<pre>template<class T, class U>
bool operator!=(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws</pre>
<blockquote>
<p><b>Returns:</b> <code>a.get() != b.get()</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<pre>template<class T, class U>
bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b); // never throws</pre>
<blockquote>
<p><b>Returns:</b> an unspecified value such that</p>
<UL>
<LI>
<b>operator<</b> is a strict weak ordering as described in section 25.3 <code>[lib.alg.sorting]</code>
of the C++ standard;
<LI>
under the equivalence relation defined by <STRONG>operator<</STRONG>, <code>!(a
< b) && !(b < a)</code>, two <STRONG>shared_ptr</STRONG> instances
are equivalent if and only if they <EM>share ownership</EM> or are both <EM>empty</EM>.</LI></UL>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> Allows <STRONG>shared_ptr</STRONG> objects to be used as keys in
associative containers.</P>
</blockquote>
<P><EM>[<STRONG>Operator<</STRONG> has been preferred over a <STRONG>std::less </STRONG>
specialization for consistency and legality reasons, as <STRONG>std::less</STRONG>
is required to return the results of <STRONG>operator<</STRONG>, and many
standard algorithms use <STRONG>operator<</STRONG> instead of <STRONG>std::less</STRONG>
for comparisons when a predicate is not supplied. Composite objects, like <STRONG>std::pair</STRONG>,
also implement their <STRONG>operator<</STRONG> in terms of their contained
subobjects' <STRONG>operator<</STRONG>.</EM></P>
<P><EM>The rest of the comparison operators are omitted by design.]</EM></P>
<h3><a name="free-swap">swap</a></h3>
<pre>template<class T>
void swap(shared_ptr<T> & a, shared_ptr<T> & b); // never throws</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>a.swap(b)</code>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> Matches the interface of <B>std::swap</B>. Provided as an aid to
generic programming.</P>
</BLOCKQUOTE>
<P><EM>[<STRONG>swap</STRONG> is defined in the same namespace as <STRONG>shared_ptr</STRONG>
as this is currently the only legal way to supply a <STRONG>swap</STRONG> function
that has a chance to be used by the standard library.]</EM></P>
<h3><a name="get_pointer">get_pointer</a></h3>
<pre>template<class T>
T * get_pointer(shared_ptr<T> const & p); // never throws</pre>
<BLOCKQUOTE>
<P><B>Returns:</B> <code>p.get()</code>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> Provided as an aid to generic programming. Used by <A href="../bind/mem_fn.html">
mem_fn</A>.</P>
</BLOCKQUOTE>
<h3><a name="static_pointer_cast">static_pointer_cast</a></h3>
<pre>template<class T, class U>
shared_ptr<T> static_pointer_cast(shared_ptr<U> const & r); // never throws</pre>
<BLOCKQUOTE>
<P><STRONG>Requires:</STRONG> The expression <code>static_cast<T*>(r.get())</code>
must be well-formed.</P>
<P><B>Returns:</B> If <b>r</b> is <i>empty</i>, an <i>empty</i> <b>shared_ptr<T></b>;
otherwise, a <STRONG>shared_ptr<T></STRONG> object that stores a copy of <code>
static_cast<T*>(r.get())</code> and <i>shares ownership</i> with <b>r</b>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> the seemingly equivalent expression</P>
<p><code>shared_ptr<T>(static_cast<T*>(r.get()))</code></p>
<p>will eventually result in undefined behavior, attempting to delete the same
object twice.</p>
</BLOCKQUOTE>
<h3><a name="const_pointer_cast">const_pointer_cast</a></h3>
<pre>template<class T, class U>
shared_ptr<T> const_pointer_cast(shared_ptr<U> const & r); // never throws</pre>
<BLOCKQUOTE>
<P><STRONG>Requires:</STRONG> The expression <code>const_cast<T*>(r.get())</code>
must be well-formed.</P>
<P><B>Returns:</B> If <b>r</b> is <i>empty</i>, an <i>empty</i> <b>shared_ptr<T></b>;
otherwise, a <STRONG>shared_ptr<T></STRONG> object that stores a copy of <code>
const_cast<T*>(r.get())</code> and <i>shares ownership</i> with <b>r</b>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> the seemingly equivalent expression</P>
<p><code>shared_ptr<T>(const_cast<T*>(r.get()))</code></p>
<p>will eventually result in undefined behavior, attempting to delete the same
object twice.</p>
</BLOCKQUOTE>
<h3><a name="dynamic_pointer_cast">dynamic_pointer_cast</a></h3>
<pre>template<class T, class U>
shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const & r);</pre>
<BLOCKQUOTE>
<P><STRONG>Requires:</STRONG> The expression <CODE>dynamic_cast<T*>(r.get())</CODE>
must be well-formed and its behavior defined.</P>
<P><B>Returns:</B></P>
<UL>
<LI>
When <CODE>dynamic_cast<T*>(r.get())</CODE> returns a nonzero value, a <STRONG>
shared_ptr<T></STRONG> object that stores a copy of it and <i>shares
ownership</i> with <STRONG>r</STRONG>;
<LI>
Otherwise, an <i>empty</i> <STRONG>shared_ptr<T></STRONG> object.</LI></UL>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> the seemingly equivalent expression</P>
<P><CODE>shared_ptr<T>(dynamic_cast<T*>(r.get()))</CODE></P>
<P>will eventually result in undefined behavior, attempting to delete the same
object twice.</P>
</BLOCKQUOTE>
<h3><a name="insertion-operator">operator<<</a></h3>
<pre>template<class E, class T, class Y>
std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);</pre>
<BLOCKQUOTE>
<p><STRONG>Effects:</STRONG> <code>os << p.get();</code>.</p>
<P><B>Returns:</B> <b>os</b>.</P>
</BLOCKQUOTE>
<h3><a name="get_deleter">get_deleter</a></h3>
<pre>template<class D, class T>
D * get_deleter(shared_ptr<T> const & p);</pre>
<BLOCKQUOTE>
<P><B>Returns:</B> If <STRONG>*this</STRONG> <EM>owns</EM> a deleter <STRONG>d</STRONG>
of type (cv-unqualified) <STRONG>D</STRONG>, returns <code>&d</code>;
otherwise returns 0.</P>
<P><B>Throws:</B> nothing.</P>
</BLOCKQUOTE>
<h2><a name="example">Example</a></h2>
<p>See <A href="example/shared_ptr_example.cpp">shared_ptr_example.cpp</A> for a
complete example program. The program builds a <b>std::vector</b> and <b>std::set</b>
of <b>shared_ptr</b> objects.</p>
<p>Note that after the containers have been populated, some of the <b>shared_ptr</b>
objects will have a use count of 1 rather than a use count of 2, since the set
is a <b>std::set</b> rather than a <b>std::multiset</b>, and thus does not
contain duplicate entries. Furthermore, the use count may be even higher at
various times while <b>push_back</b> and <b>insert</b> container operations are
performed. More complicated yet, the container operations may throw exceptions
under a variety of circumstances. Getting the memory management and exception
handling in this example right without a smart pointer would be a nightmare.</p>
<h2><a name="Handle/Body">Handle/Body</a> Idiom</h2>
<p>One common usage of <b>shared_ptr</b> is to implement a handle/body (also called
pimpl) idiom which avoids exposing the body (implementation) in the header
file.</p>
<p>The <A href="example/shared_ptr_example2_test.cpp">shared_ptr_example2_test.cpp</A>
sample program includes a header file, <A href="example/shared_ptr_example2.hpp">shared_ptr_example2.hpp</A>,
which uses a <b>shared_ptr<></b> to an incomplete type to hide the
implementation. The instantiation of member functions which require a complete
type occurs in the <A href="example/shared_ptr_example2.cpp">shared_ptr_example2.cpp</A>
implementation file. Note that there is no need for an explicit destructor.
Unlike ~scoped_ptr, ~shared_ptr does not require that <b>T</b> be a complete
type.</p>
<h2><a name="ThreadSafety">Thread Safety</a></h2>
<p><STRONG>shared_ptr</STRONG> objects offer the same level of thread safety as
built-in types. A <STRONG>shared_ptr</STRONG> instance can be "read" (accessed
using only const operations) simultaneously by multiple threads. Different <STRONG>shared_ptr</STRONG>
instances can be "written to" (accessed using mutable operations such as <STRONG>operator=
</STRONG>or <STRONG>reset</STRONG>) simultaneosly by multiple threads (even
when these instances are copies, and share the same reference count
underneath.)</p>
<P>Any other simultaneous accesses result in undefined behavior.</P>
<P>Examples:</P>
<pre>shared_ptr<int> p(new int(42));
//--- Example 1 ---
// thread A
shared_ptr<int> p2(p); // reads p
// thread B
shared_ptr<int> p3(p); // OK, multiple reads are safe
//--- Example 2 ---
// thread A
p.reset(new int(1912)); // writes p
// thread B
p2.reset(); // OK, writes p2
//--- Example 3 ---
// thread A
p = p3; // reads p3, writes p
// thread B
p3.reset(); // writes p3; undefined, simultaneous read/write
//--- Example 4 ---
// thread A
p3 = p2; // reads p2, writes p3
// thread B
// p2 goes out of scope: undefined, the destructor is considered a "write access"
//--- Example 5 ---
// thread A
p3.reset(new int(1));
// thread B
p3.reset(new int(2)); // undefined, multiple writes
</pre>
<p> </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>
GNU GCC on IA64;
<LI>
Metrowerks CodeWarrior on PowerPC;
<LI>
GNU GCC on PowerPC;
<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
implementation? It would be useful to be able to experiment with each type so
as to find the most suitable for the job at hand?</P>
<P>
<b>A.</b> An important goal of <STRONG>shared_ptr</STRONG> is to provide a
standard shared-ownership pointer. Having a single pointer type is important
for stable library interfaces, since different shared pointers typically cannot
interoperate, i.e. a reference counted pointer (used by library A) cannot share
ownership with a linked pointer (used by library B.)<BR>
</P>
<P><B>Q.</B> Why doesn't <B>shared_ptr</B> have template parameters supplying
traits or policies to allow extensive user customization?</P>
<P>
<B>A.</B> Parameterization discourages users. The <B>shared_ptr</B> template is
carefully crafted to meet common needs without extensive parameterization. Some
day a highly configurable smart pointer may be invented that is also very easy
to use and very hard to misuse. Until then, <B>shared_ptr</B> is the smart
pointer of choice for a wide range of applications. (Those interested in policy
based smart pointers should read <A href="http://www.awprofessional.com/bookstore/product.asp?isbn=0201704315&rl=1">
Modern C++ Design</A> by Andrei Alexandrescu.)<BR>
</P>
<P><B>Q.</B> I am not convinced. Default parameters can be used where appropriate
to hide the complexity. Again, why not policies?</P>
<P>
<B>A.</B> Template parameters affect the type. See the answer to the first
question above.<BR>
</P>
<P><B>Q.</B> Why doesn't <b>shared_ptr</b> use a linked list implementation?</P>
<P>
<b>A.</b> A linked list implementation does not offer enough advantages to
offset the added cost of an extra pointer. See <A href="smarttests.htm">timings</A>
page. In addition, it is expensive to make a linked list implementation thread
safe.<BR>
</P>
<P><b>Q.</b> Why doesn't <b>shared_ptr</b> (or any of the other Boost smart
pointers) supply an automatic conversion to <b>T*</b>?</P>
<P>
<b>A.</b> Automatic conversion is believed to be too error prone.<BR>
</P>
<P><B>Q.</B> Why does <b>shared_ptr</b> supply use_count()?</P>
<P>
<b>A.</b> As an aid to writing test cases and debugging displays. One of the
progenitors had use_count(), and it was useful in tracking down bugs in a
complex project that turned out to have cyclic-dependencies.<BR>
</P>
<P><B>Q.</B> Why doesn't <b>shared_ptr</b> specify complexity requirements?</P>
<P>
<b>A.</b> Because complexity requirements limit implementors and complicate the
specification without apparent benefit to <b>shared_ptr</b> users. For example,
error-checking implementations might become non-conforming if they had to meet
stringent complexity requirements.<BR>
</P>
<P><b>Q.</b> Why doesn't <b>shared_ptr</b> provide a release() function?</P>
<P>
<b>A.</b> <b>shared_ptr</b> cannot give away ownership unless it's unique()
because the other copy will still destroy the object.</P>
<p>Consider:</p>
<blockquote><pre>shared_ptr<int> a(new int);
shared_ptr<int> b(a); // a.use_count() == b.use_count() == 2
int * p = a.release();
// Who owns p now? b will still call delete on it in its destructor.</pre>
</blockquote>
<p>Furthermore, the pointer returned by <code>release()</code> would be difficult
to deallocate reliably, as the source <b>shared_ptr</b> could have been created
with a custom deleter.<BR>
</p>
<P><b>Q.</b> Why is <code>operator->()</code> const, but its return value is a
non-const pointer to the element type?</P>
<P>
<b>A.</b> Shallow copy pointers, including raw pointers, typically don't
propagate constness. It makes little sense for them to do so, as you can always
obtain a non-const pointer from a const one and then proceed to modify the
object through it.<b>shared_ptr</b> is "as close to raw pointers as possible
but no closer".<BR>
<BR>
</P>
<hr>
<p>
$Date: 2009-03-11 11:08:14 -0400 (Wed, 11 Mar 2009) $</p>
<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Copyright 2002-2005 Peter Dimov. 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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