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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>weak_ptr class template</h1>
<p><A href="#Introduction">Introduction</A><br>
<A href="#Synopsis">Synopsis</A><br>
<A href="#Members">Members</A><br>
<A href="#functions">Free Functions</A><br>
<A href="#FAQ">Frequently Asked Questions</A>
</p>
<h2><a name="Introduction">Introduction</a></h2>
<p>The <b>weak_ptr</b> class template stores a "weak reference" to an object that's
already managed by a <b>shared_ptr</b>. To access the object, a <STRONG>weak_ptr</STRONG>
can be converted to a <STRONG>shared_ptr</STRONG> using <A href="shared_ptr.htm#constructors">
the <STRONG>shared_ptr</STRONG> constructor</A> or the member function <STRONG><A href="#lock">
lock</A></STRONG>. When the last <b>shared_ptr</b> to the object goes
away and the object is deleted, the attempt to obtain a <STRONG>shared_ptr</STRONG>
from the <b>weak_ptr</b> instances that refer to the deleted object will fail:
the constructor will throw an exception of type <STRONG>boost::bad_weak_ptr</STRONG>,
and <STRONG>weak_ptr::lock</STRONG> will return an <EM>empty</EM> <STRONG>shared_ptr</STRONG>.</p>
<p>Every <b>weak_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>weak_ptr</b> works with the
standard library's associative containers.</p>
<P><STRONG>weak_ptr</STRONG> operations never throw exceptions.</P>
<p>The class template is parameterized on <b>T</b>, the type of the object pointed
to.</p>
<P>Compared to <STRONG>shared_ptr</STRONG>, <STRONG>weak_ptr</STRONG> provides a
very limited subset of operations since accessing its stored pointer is often
dangerous in multithreaded programs, and sometimes unsafe even within a single
thread (that is, it may invoke undefined behavior.) Pretend for a moment that <b>weak_ptr</b>
has a <b>get</b> member function that returns a raw pointer, and consider this
innocent piece of code:</P>
<pre>shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);
// some time later
if(int * r = q.get())
{
// use *r
}
</pre>
<P>Imagine that after the <STRONG>if</STRONG>, but immediately before <STRONG>r</STRONG>
is used, another thread executes the statement <code>p.reset()</code>. Now <STRONG>r</STRONG>
is a dangling pointer.</P>
<P>The solution to this problem is to create a temporary <STRONG>shared_ptr</STRONG>
from <STRONG>q</STRONG>:</P>
<pre>shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);
// some time later
if(shared_ptr<int> r = q.<A href="#lock" >lock</A>())
{
// use *r
}
</pre>
<p>Now <STRONG>r</STRONG> holds a reference to the object that was pointed by <STRONG>q</STRONG>.
Even if <code>p.reset()</code> is executed in another thread, the object will
stay alive until <STRONG>r</STRONG> goes out of scope or is reset. By obtaining
a <STRONG>shared_ptr</STRONG> to the object, we have effectively locked it
against destruction.</p>
<h2><a name="Synopsis">Synopsis</a></h2>
<pre>namespace boost {
template<class T> class weak_ptr {
public:
typedef T <A href="#element_type" >element_type</A>;
<A href="#default-constructor" >weak_ptr</A>();
template<class Y> <A href="#constructors" >weak_ptr</A>(shared_ptr<Y> const & r);
<A href="#constructors" >weak_ptr</A>(weak_ptr const & r);
template<class Y> <A href="#constructors" >weak_ptr</A>(weak_ptr<Y> const & r);
<A href="#destructor" >~weak_ptr</A>();
weak_ptr & <A href="#assignment" >operator=</A>(weak_ptr const & r);
template<class Y> weak_ptr & <A href="#assignment" >operator=</A>(weak_ptr<Y> const & r);
template<class Y> weak_ptr & <A href="#assignment" >operator=</A>(shared_ptr<Y> const & r);
long <A href="#use_count" >use_count</A>() const;
bool <A href="#expired" >expired</A>() const;
shared_ptr<T> <A href="#lock" >lock</A>() const;
void <A href="#reset" >reset</A>();
void <A href="#swap" >swap</A>(weak_ptr<T> & b);
};
template<class T, class U>
bool <A href="#comparison" >operator<</A>(weak_ptr<T> const & a, weak_ptr<U> const & b);
template<class T>
void <A href="#free-swap" >swap</A>(weak_ptr<T> & a, weak_ptr<T> & b);
}
</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="default-constructor">constructors</a></h3>
<pre>weak_ptr();</pre>
<blockquote>
<p><b>Effects:</b> Constructs an <EM>empty</EM> <b>weak_ptr</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == 0</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote><a name="constructors"></a>
<pre>template<class Y> weak_ptr</A>(shared_ptr<Y> const & r);
weak_ptr(weak_ptr const & r);
template<class Y> weak_ptr(weak_ptr<Y> const & r);</pre>
<blockquote>
<p><b>Effects:</b> If <STRONG>r</STRONG> is <EM>empty</EM>, constructs an <EM>empty</EM>
<STRONG>weak_ptr</STRONG>; otherwise, constructs a <b>weak_ptr</b> that <EM>shares
ownership</EM> with <STRONG>r</STRONG> as if by storing a copy of the
pointer stored in <b>r</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == r.use_count()</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<h3><a name="destructor">destructor</a></h3>
<pre>~weak_ptr();</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Destroys this <b>weak_ptr</b> but has no effect on the object
its stored pointer points to.</P>
<P><B>Throws:</B> nothing.</P>
</BLOCKQUOTE>
<h3><a name="assignment">assignment</a></h3>
<pre>weak_ptr & operator=(weak_ptr const & r);
template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r);
template<class Y> weak_ptr & operator=(shared_ptr<Y> const & r);</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>weak_ptr(r).swap(*this)</code>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> The implementation is free to meet the effects (and the implied
guarantees) via different means, without creating a temporary.</P>
</BLOCKQUOTE>
<h3><a name="use_count">use_count</a></h3>
<pre>long use_count() const;</pre>
<blockquote>
<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>
</blockquote>
<h3><a name="expired">expired</a></h3>
<pre>bool expired() const;</pre>
<blockquote>
<p><b>Returns:</b> <code>use_count() == 0</code>.</p>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> <code>expired()</code> may be faster than <code>use_count()</code>.</P>
</blockquote>
<h3><a name="lock">lock</a></h3>
<pre>shared_ptr<T> lock() const;</pre>
<BLOCKQUOTE>
<P><B>Returns:</B> <code>expired()? shared_ptr<T>(): shared_ptr<T>(*this)</code>.</P>
<P><B>Throws:</B> nothing.</P>
</BLOCKQUOTE>
<h3><a name="reset">reset</a></h3>
<pre>void reset();</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>weak_ptr().swap(*this)</code>.</P>
</BLOCKQUOTE>
<h3><a name="swap">swap</a></h3>
<pre>void swap(weak_ptr & b);</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<(weak_ptr<T> const & a, weak_ptr<U> const & b);</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>weak_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>weak_ptr</STRONG> objects to be used as keys in
associative containers.</P>
</blockquote>
<h3><a name="free-swap">swap</a></h3>
<pre>template<class T>
void swap(weak_ptr<T> & a, weak_ptr<T> & b)</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>
<h2><a name="FAQ">Frequently Asked Questions</a></h2>
<P><B>Q.</B> Can an object create a <STRONG>weak_ptr</STRONG> to itself in its
constructor?</P>
<P><b>A.</b> No. A <STRONG>weak_ptr</STRONG> can only be created from a <STRONG>shared_ptr</STRONG>,
and at object construction time no <STRONG>shared_ptr</STRONG> to the object
exists yet. Even if you could create a temporary <STRONG>shared_ptr</STRONG> to <STRONG>
this</STRONG>, it would go out of scope at the end of the constructor, and
all <STRONG>weak_ptr</STRONG> instances would instantly expire.</P>
<P>The solution is to make the constructor private, and supply a factory function
that returns a <STRONG>shared_ptr</STRONG>:<BR>
</P>
<pre>
class X
{
private:
X();
public:
static shared_ptr<X> create()
{
shared_ptr<X> px(new X);
// create weak pointers from px here
return px;
}
};
</pre>
<p><br>
</p>
<hr>
<p>$Date: 2007-11-25 13:38:02 -0500 (Sun, 25 Nov 2007) $</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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