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<title>weak_ptr</title>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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2003-01-04 14:24:14 +00:00
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<body text="#000000" bgColor="#ffffff">
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2004-10-05 15:45:52 +00:00
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<h1><IMG height="86" alt="boost.png (6897 bytes)" src="../../boost.png" width="277" align="middle">weak_ptr
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class template</h1>
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<p><A href="#Introduction">Introduction</A><br>
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<A href="#Synopsis">Synopsis</A><br>
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<A href="#Members">Members</A><br>
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<A href="#functions">Free Functions</A><br>
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<A href="#FAQ">Frequently Asked Questions</A>
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</p>
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<h2><a name="Introduction">Introduction</a></h2>
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<p>The <b>weak_ptr</b> class template stores a "weak reference" to an object that's
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already managed by a <b>shared_ptr</b>. To access the object, a <STRONG>weak_ptr</STRONG>
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can be converted to a <STRONG>shared_ptr</STRONG> using <A href="shared_ptr.htm#constructors">
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the <STRONG>shared_ptr</STRONG> constructor</A> or the member function <STRONG><A href="#lock">
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lock</A></STRONG>. When the last <b>shared_ptr</b> to the object goes
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away and the object is deleted, the attempt to obtain a <STRONG>shared_ptr</STRONG>
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from the <b>weak_ptr</b> instances that refer to the deleted object will fail:
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the constructor will throw an exception of type <STRONG>boost::bad_weak_ptr</STRONG>,
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and <STRONG>weak_ptr::lock</STRONG> will return an <EM>empty</EM> <STRONG>shared_ptr</STRONG>.</p>
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<p>Every <b>weak_ptr</b> meets the <b>CopyConstructible</b> and <b>Assignable</b> requirements
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of the C++ Standard Library, and so can be used in standard library containers.
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Comparison operators are supplied so that <b>weak_ptr</b> works with the
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standard library's associative containers.</p>
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<P><STRONG>weak_ptr</STRONG> operations never throw exceptions.</P>
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<p>The class template is parameterized on <b>T</b>, the type of the object pointed
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to.</p>
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<P>Compared to <STRONG>shared_ptr</STRONG>, <STRONG>weak_ptr</STRONG> provides a
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very limited subset of operations since accessing its stored pointer is often
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dangerous in multithreaded programs, and sometimes unsafe even within a single
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thread (that is, it may invoke undefined behavior.) Pretend for a moment that <b>weak_ptr</b>
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has a <b>get</b> member function that returns a raw pointer, and consider this
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innocent piece of code:</P>
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<pre>shared_ptr<int> p(new int(5));
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weak_ptr<int> q(p);
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// some time later
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if(int * r = q.get())
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{
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// use *r
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}
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</pre>
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<P>Imagine that after the <STRONG>if</STRONG>, but immediately before <STRONG>r</STRONG>
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is used, another thread executes the statement <code>p.reset()</code>. Now <STRONG>r</STRONG>
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is a dangling pointer.</P>
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<P>The solution to this problem is to create a temporary <STRONG>shared_ptr</STRONG>
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from <STRONG>q</STRONG>:</P>
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<pre>shared_ptr<int> p(new int(5));
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weak_ptr<int> q(p);
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// some time later
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if(shared_ptr<int> r = q.<A href="#lock" >lock</A>())
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{
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// use *r
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}
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</pre>
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<p>Now <STRONG>r</STRONG> holds a reference to the object that was pointed by <STRONG>q</STRONG>.
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Even if <code>p.reset()</code> is executed in another thread, the object will
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stay alive until <STRONG>r</STRONG> goes out of scope or is reset. By obtaining
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a <STRONG>shared_ptr</STRONG> to the object, we have effectively locked it
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against destruction.</p>
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<h2><a name="Synopsis">Synopsis</a></h2>
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<pre>namespace boost {
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template<class T> class weak_ptr {
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public:
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typedef T <A href="#element_type" >element_type</A>;
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<A href="#default-constructor" >weak_ptr</A>();
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template<class Y> <A href="#constructors" >weak_ptr</A>(shared_ptr<Y> const & r);
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<A href="#constructors" >weak_ptr</A>(weak_ptr const & r);
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template<class Y> <A href="#constructors" >weak_ptr</A>(weak_ptr<Y> const & r);
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<A href="#destructor" >~weak_ptr</A>();
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weak_ptr & <A href="#assignment" >operator=</A>(weak_ptr const & r);
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template<class Y> weak_ptr & <A href="#assignment" >operator=</A>(weak_ptr<Y> const & r);
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template<class Y> weak_ptr & <A href="#assignment" >operator=</A>(shared_ptr<Y> const & r);
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long <A href="#use_count" >use_count</A>() const;
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bool <A href="#expired" >expired</A>() const;
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shared_ptr<T> <A href="#lock" >lock</A>() const;
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void <A href="#reset" >reset</A>();
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void <A href="#swap" >swap</A>(weak_ptr<T> & b);
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};
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template<class T, class U>
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bool <A href="#comparison" >operator<</A>(weak_ptr<T> const & a, weak_ptr<U> const & b);
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template<class T>
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void <A href="#free-swap" >swap</A>(weak_ptr<T> & a, weak_ptr<T> & b);
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}
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</pre>
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<h2><a name="Members">Members</a></h2>
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<h3><a name="element_type">element_type</a></h3>
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<pre>typedef T element_type;</pre>
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<blockquote>
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<p>Provides the type of the template parameter T.</p>
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</blockquote>
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<h3><a name="default-constructor">constructors</a></h3>
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<pre>weak_ptr();</pre>
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<blockquote>
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<p><b>Effects:</b> Constructs an <EM>empty</EM> <b>weak_ptr</b>.</p>
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<p><b>Postconditions:</b> <code>use_count() == 0</code>.</p>
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<p><b>Throws:</b> nothing.</p>
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</blockquote><a name="constructors"></a>
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<pre>template<class Y> weak_ptr</A>(shared_ptr<Y> const & r);
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weak_ptr(weak_ptr const & r);
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template<class Y> weak_ptr(weak_ptr<Y> const & r);</pre>
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<blockquote>
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<p><b>Effects:</b> If <STRONG>r</STRONG> is <EM>empty</EM>, constructs an <EM>empty</EM>
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<STRONG>weak_ptr</STRONG>; otherwise, constructs a <b>weak_ptr</b> that <EM>shares
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ownership</EM> with <STRONG>r</STRONG> as if by storing a copy of the
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pointer stored in <b>r</b>.</p>
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<p><b>Postconditions:</b> <code>use_count() == r.use_count()</code>.</p>
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<p><b>Throws:</b> nothing.</p>
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</blockquote>
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<h3><a name="destructor">destructor</a></h3>
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<pre>~weak_ptr();</pre>
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<BLOCKQUOTE>
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<P><B>Effects:</B> Destroys this <b>weak_ptr</b> but has no effect on the object
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its stored pointer points to.</P>
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<P><B>Throws:</B> nothing.</P>
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</BLOCKQUOTE>
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<h3><a name="assignment">assignment</a></h3>
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<pre>weak_ptr & operator=(weak_ptr const & r);
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template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r);
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template<class Y> weak_ptr & operator=(shared_ptr<Y> const & r);</pre>
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<BLOCKQUOTE>
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<P><B>Effects:</B> Equivalent to <code>weak_ptr(r).swap(*this)</code>.</P>
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<P><B>Throws:</B> nothing.</P>
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<P><B>Notes:</B> The implementation is free to meet the effects (and the implied
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guarantees) via different means, without creating a temporary.</P>
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</BLOCKQUOTE>
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<h3><a name="use_count">use_count</a></h3>
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<pre>long use_count() const;</pre>
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<blockquote>
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<p><b>Returns:</b> if <STRONG>*this</STRONG> is <EM>empty</EM>, an unspecified
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nonnegative value; otherwise, the number of <b>shared_ptr</b> objects that <EM>share
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ownership</EM> with <STRONG>*this</STRONG>.</p>
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<p><b>Throws:</b> nothing.</p>
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<P><B>Notes:</B> <code>use_count()</code> is not necessarily efficient. Use only
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for debugging and testing purposes, not for production code.</P>
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</blockquote>
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<h3><a name="expired">expired</a></h3>
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<pre>bool expired() const;</pre>
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<blockquote>
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<p><b>Returns:</b> <code>use_count() == 0</code>.</p>
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<p><b>Throws:</b> nothing.</p>
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<P><B>Notes:</B> <code>expired()</code> may be faster than <code>use_count()</code>.</P>
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</blockquote>
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<h3><a name="lock">lock</a></h3>
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<pre>shared_ptr<T> lock() const;</pre>
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<BLOCKQUOTE>
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<P><B>Returns:</B> <code>expired()? shared_ptr<T>(): shared_ptr<T>(*this)</code>.</P>
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<P><B>Throws:</B> nothing.</P>
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</BLOCKQUOTE>
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<h3><a name="reset">reset</a></h3>
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<pre>void reset();</pre>
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<BLOCKQUOTE>
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<P><B>Effects:</B> Equivalent to <code>weak_ptr().swap(*this)</code>.</P>
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</BLOCKQUOTE>
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<h3><a name="swap">swap</a></h3>
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<pre>void swap(weak_ptr & b);</pre>
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<blockquote>
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<p><b>Effects:</b> Exchanges the contents of the two smart pointers.</p>
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<p><b>Throws:</b> nothing.</p>
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</blockquote>
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<h2><a name="functions">Free Functions</a></h2>
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<h3><a name="comparison">comparison</a></h3>
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<pre>template<class T, class U>
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bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b);</pre>
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<blockquote>
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<p><b>Returns:</b> an unspecified value such that</p>
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<UL>
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<LI>
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<b>operator<</b> is a strict weak ordering as described in section 25.3 <code>[lib.alg.sorting]</code>
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of the C++ standard;
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<LI>
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under the equivalence relation defined by <STRONG>operator<</STRONG>, <code>!(a
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< b) && !(b < a)</code>, two <STRONG>weak_ptr</STRONG> instances
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are equivalent if and only if they <EM>share ownership</EM>.</LI></UL>
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<p><b>Throws:</b> nothing.</p>
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<P><B>Notes:</B> Allows <STRONG>weak_ptr</STRONG> objects to be used as keys in
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associative containers.</P>
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</blockquote>
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<h3><a name="free-swap">swap</a></h3>
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<pre>template<class T>
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void swap(weak_ptr<T> & a, weak_ptr<T> & b)</pre>
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<BLOCKQUOTE>
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<P><B>Effects:</B> Equivalent to <code>a.swap(b)</code>.</P>
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<P><B>Throws:</B> nothing.</P>
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<P><B>Notes:</B> Matches the interface of <B>std::swap</B>. Provided as an aid to
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generic programming.</P>
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</BLOCKQUOTE>
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<h2><a name="FAQ">Frequently Asked Questions</a></h2>
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<P><B>Q.</B> Can an object create a <STRONG>weak_ptr</STRONG> to itself in its
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constructor?</P>
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<P><b>A.</b> No. A <STRONG>weak_ptr</STRONG> can only be created from a <STRONG>shared_ptr</STRONG>,
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and at object construction time no <STRONG>shared_ptr</STRONG> to the object
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exists yet. Even if you could create a temporary <STRONG>shared_ptr</STRONG> to <STRONG>
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this</STRONG>, it would go out of scope at the end of the constructor, and
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all <STRONG>weak_ptr</STRONG> instances would instantly expire.</P>
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<P>The solution is to make the constructor private, and supply a factory function
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that returns a <STRONG>shared_ptr</STRONG>:<BR>
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</P>
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<pre>
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class X
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{
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private:
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X();
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public:
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static shared_ptr<X> create()
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{
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shared_ptr<X> px(new X);
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// create weak pointers from px here
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return px;
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}
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};
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</pre>
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2003-01-13 18:32:16 +00:00
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<p><br>
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</p>
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2002-03-08 16:56:16 +00:00
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<hr>
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2003-01-04 14:24:14 +00:00
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<p>$Date$</p>
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2003-02-05 12:56:48 +00:00
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<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
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2003-02-06 14:54:15 +00:00
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Copyright 2002, 2003 Peter Dimov. Permission to copy, use, modify, sell and
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distribute this document is granted provided this copyright notice appears in
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all copies. This document is provided "as is" without express or implied
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warranty, and with no claim as to its suitability for any purpose.</small></p>
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2002-03-08 16:56:16 +00:00
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</A>
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</body>
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2002-02-02 18:36:12 +00:00
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</html>
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