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Casts removed as unsafe, added intro paragraph about make_shared.
[SVN r13180]
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Peter Dimov
99
weak_ptr.htm
99
weak_ptr.htm
@ -18,6 +18,40 @@
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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. <b>T</b> must meet the smart pointer <a href="smart_ptr.htm#Common requirements">
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common requirements</a>.</p>
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<P>Compared to <STRONG>shared_ptr</STRONG>, <STRONG>weak_ptr</STRONG> provides
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a very limited subset of operations since accessing its stored pointer is
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unsafe in multithreaded programs (that is, it may invoke undefined
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behavior.) Consider, for example, this innocent piece of code:</P>
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<pre>
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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>
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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 = <a href="#make_shared">make_shared</a>(q))
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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.)</p>
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<h2><a name="Synopsis">Synopsis</a></h2>
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<pre>namespace boost {
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@ -58,15 +92,6 @@
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template<typename T>
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shared_ptr<T> <a href="#make_shared">make_shared</a>(weak_ptr<T> const & r); // never throws
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_static_cast">shared_static_cast</a>(weak_ptr<U> const & r); // never throws
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_dynamic_cast">shared_dynamic_cast</a>(weak_ptr<U> const & r);
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_polymorphic_cast">shared_polymorphic_cast</a>(weak_ptr<U> const & r);
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template<typename T, typename U>
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weak_ptr<T> <a href="#shared_polymorphic_downcast">shared_polymorphic_downcast</a>(weak_ptr<U> const & r); // never throws
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}
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</pre>
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<h2><a name="Members">Members</a></h2>
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@ -206,62 +231,8 @@ template<typename T, typename U>
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<P><B>Returns:</B> <code>r.expired()? shared_ptr<T>(): shared_ptr<T>(r)</code>.</P>
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<P><B>Throws:</B> nothing.</P>
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</BLOCKQUOTE>
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<h3><a name="shared_static_cast">shared_static_cast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_static_cast(weak_ptr<U> const & r); // never throws</pre>
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<BLOCKQUOTE>
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<P><STRONG>Requires:</STRONG> The expression <code>static_cast<T*>(r.get())</code>
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must be well-formed.</P>
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<P><B>Returns:</B> A <STRONG>weak_ptr<T></STRONG> object that stores a copy
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of <code>static_cast<T*>(r.get())</code> and shares ownership with <b>r</b>.</P>
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<P><B>Throws:</B> nothing.</P>
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</BLOCKQUOTE>
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<h3><a name="shared_dynamic_cast">shared_dynamic_cast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_dynamic_cast(weak_ptr<U> const & r);</pre>
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<BLOCKQUOTE>
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<P><STRONG>Requires:</STRONG> The expression <CODE>dynamic_cast<T*>(r.get())</CODE>
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must be well-formed and its behavior defined.</P>
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<P><B>Returns:</B></P>
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<UL>
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<LI>
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When <CODE>dynamic_cast<T*>(r.get())</CODE> returns a nonzero
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value, a <STRONG>weak_ptr<T></STRONG> object that stores a copy of
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it and shares ownership with <STRONG>r</STRONG>;
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<LI>
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Otherwise, a default-constructed <STRONG>weak_ptr<T></STRONG> object.</LI></UL>
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<P><B>Throws:</B> <STRONG>std::bad_alloc</STRONG>.</P>
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<P><B>Exception safety:</B> If an exception is thrown, the function has no effect.</P>
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</BLOCKQUOTE>
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<h3><a name="shared_polymorphic_cast">shared_polymorphic_cast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_polymorphic_cast(weak_ptr<U> const & r);</pre>
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<BLOCKQUOTE>
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<p><STRONG>Requires:</STRONG> The expression <CODE><A href="../conversion/cast.htm#Polymorphic_cast">
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polymorphic_cast</A><T*>(r.get())</CODE> must be well-formed and
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its behavior defined.</p>
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<P><B>Returns:</B> A <STRONG>weak_ptr<T></STRONG> object that stores a copy
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of <CODE><A href="../conversion/cast.htm#Polymorphic_cast">polymorphic_cast</A><T*>(r.get())</CODE>
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and shares ownership with <B>r</B>.</P>
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<P><B>Throws:</B> <STRONG>std::bad_cast</STRONG> when the pointer cannot be
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converted.</P>
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<P><B>Exception safety:</B> If an exception is thrown, the function has no effect.</P>
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</BLOCKQUOTE>
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<h3><a name="shared_polymorphic_downcast">shared_polymorphic_downcast</a></h3>
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<pre>template<typename T, typename U>
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weak_ptr<T> shared_polymorphic_downcast(weak_ptr<U> const & r); // never throws</pre>
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<BLOCKQUOTE>
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<p><STRONG>Requires:</STRONG> The expression <CODE><A href="../conversion/cast.htm#Polymorphic_cast">
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polymorphic_downcast</A><T*>(r.get())</CODE> must be well-formed
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and its behavior defined.</p>
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<P><B>Returns:</B> A <STRONG>weak_ptr<T></STRONG> object that stores a copy
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of <CODE><A href="../conversion/cast.htm#Polymorphic_cast">polymorphic_downcast</A><T*>(r.get())</CODE>
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and shares ownership with <B>r</B>.</P>
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<P><B>Throws:</B> nothing.</P>
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</BLOCKQUOTE>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B %Y" startspan -->
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8 March 2002<!--webbot bot="Timestamp" i-checksum="38439" endspan --></p>
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<p>Revised 12 March 2002<!--webbot bot="Timestamp" i-checksum="38439" endspan --></p>
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<p>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
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Copyright 2002 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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