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210 lines
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210 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)">
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<meta name="KEYWORDS" content="HOWTO, libstdc++, GCC, g++, libg++, STL">
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<meta name="DESCRIPTION" content="HOWTO for the libstdc++ chapter 20.">
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<meta name="GENERATOR" content="vi and eight fingers">
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<title>libstdc++-v3 HOWTO: Chapter 20</title>
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<link rel="StyleSheet" href="../lib3styles.css">
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</head>
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<body>
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<h1 class="centered"><a name="top">Chapter 20: General Utilities</a></h1>
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<p>Chapter 20 deals with utility classes and functions, such as
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the oft-debated <code>auto_ptr<></code>.
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</p>
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<!-- ####################################################### -->
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<hr>
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<h1>Contents</h1>
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<ul>
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<li><a href="#1"><code>auto_ptr</code> is not omnipotent</a>
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<li><a href="#2"><code>auto_ptr</code> inside container classes</a>
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<li><a href="#3">Functors</a>
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<li><a href="#4">Pairs</a>
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</ul>
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<hr>
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<!-- ####################################################### -->
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<h2><a name="1"><code>auto_ptr</code> is not omnipotent</a></h2>
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<p>I'm not going to try and explain all of the fun and delicious
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things that can happen with misuse of the auto_ptr class template
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(called AP here), nor am I going to try and teach you how to use
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AP safely in the presence of copying. The AP class is a really
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nifty idea for a smart pointer, but it is one of the dumbest of
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all the smart pointers -- and that's fine.
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</p>
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<p>AP is not meant to be a supersmart solution to all resource
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leaks everywhere. Neither is it meant to be an effective form
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of garbage collection (although it can help, a little bit).
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And it can <em>not</em> be used for arrays!
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</p>
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<p>AP <em>is</em> meant to prevent nasty leaks in the presence of
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exceptions. That's <em>all</em>. This code is AP-friendly:
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<pre>
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// not a recommend naming scheme, but good for web-based FAQs
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typedef std::auto_ptr<MyClass> APMC;
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extern function_taking_MyClass_pointer (MyClass*);
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extern some_throwable_function ();
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void func (int data)
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{
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APMC ap (new MyClass(data));
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some_throwable_function(); // this will throw an exception
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function_taking_MyClass_pointer (ap.get());
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}
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</pre>When an exception gets thrown, the instance of MyClass that's
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been created on the heap will be <code>delete</code>'d as the stack is
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unwound past <code>func()</code>.
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</p>
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<p>Changing that code as follows is <em>not</em> AP-friendly:
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<pre>
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APMC ap (new MyClass[22]);
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</pre>You will get the same problems as you would without the use
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of AP:
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<pre>
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char* array = new char[10]; // array new...
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...
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delete array; // ...but single-object delete
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</pre>
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</p>
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<p>AP cannot tell whether the pointer you've passed at creation points
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to one or many things. If it points to many things, you are about
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to die. AP is trivial to write, however, so you could write your
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own <code>auto_array_ptr</code> for that situation (in fact, this has
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been done many times; check the mailing lists, Usenet, Boost, etc).
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr>
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<h2><a name="2"><code>auto_ptr</code> inside container classes</a></h2>
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<p>All of the <a href="../23_containers/howto.html">containers</a>
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described in the standard library require their contained types
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to have, among other things, a copy constructor like this:
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<pre>
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struct My_Type
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{
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My_Type (My_Type const&);
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};
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</pre>
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Note the const keyword; the object being copied shouldn't change.
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The template class <code>auto_ptr</code> (called AP here) does not
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meet this requirement. Creating a new AP by copying an existing
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one transfers ownership of the pointed-to object, which means that
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the AP being copied must change, which in turn means that the
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copy ctors of AP do not take const objects.
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</p>
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<p>The resulting rule is simple: <em>Never ever use a container of
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auto_ptr objects.</em> The standard says that "undefined"
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behavior is the result, but it is guaranteed to be messy.
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</p>
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<p>To prevent you from doing this to yourself, the
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<a href="../19_diagnostics/howto.html#3">concept checks</a> built
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in to this implementation will issue an error if you try to
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compile code like this:
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<pre>
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#include <vector>
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#include <memory>
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void f()
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{
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std::vector< std::auto_ptr<int> > vec_ap_int;
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}
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</pre>
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Should you try this with the checks enabled, you will see an error.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr>
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<h2><a name="3">Functors</a></h2>
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<p>If you don't know what functors are, you're not alone. Many people
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get slightly the wrong idea. In the interest of not reinventing
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the wheel, we will refer you to the introduction to the functor
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concept written by SGI as part of their STL, in
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<a href="http://www.sgi.com/Technology/STL/functors.html">their
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http://www.sgi.com/Technology/STL/functors.html</a>.
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<hr>
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<h2><a name="4">Pairs</a></h2>
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<p>The <code>pair<T1,T2></code> is a simple and handy way to
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carry around a pair of objects. One is of type T1, and another of
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type T2; they may be the same type, but you don't get anything
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extra if they are. The two members can be accessed directly, as
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<code>.first</code> and <code>.second</code>.
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</p>
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<p>Construction is simple. The default ctor initializes each member
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with its respective default ctor. The other simple ctor,
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<pre>
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pair (const T1& x, const T2& y);
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</pre>does what you think it does, <code>first</code> getting <code>x</code>
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and <code>second</code> getting <code>y</code>.
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</p>
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<p>There is a copy constructor, but it requires that your compiler
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handle member function templates:
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<pre>
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template <class U, class V> pain (const pair<U,V>& p);
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</pre>The compiler will convert as necessary from U to T1 and from
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V to T2 in order to perform the respective initializations.
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</p>
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<p>The comparison operators are done for you. Equality
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of two <code>pair<T1,T2></code>s is defined as both <code>first</code>
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members comparing equal and both <code>second</code> members comparing
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equal; this simply delegates responsibility to the respective
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<code>operator==</code> functions (for types like MyClass) or builtin
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comparisons (for types like int, char, etc).
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</p>
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<a name="pairlt">
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<p>The less-than operator is a bit odd the first time you see it. It
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is defined as evaluating to:
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<pre>
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x.first < y.first ||
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( !(y.first < x.first) && x.second < y.second )
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</pre>
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The other operators are not defined using the <code>rel_ops</code>
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functions above, but their semantics are the same.
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</p>
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</a>
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<p>Finally, there is a template function called <code>make_pair</code>
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that takes two references-to-const objects and returns an
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instance of a pair instantiated on their respective types:
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<pre>
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pair<int,MyClass> p = make_pair(4,myobject);
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</pre>
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</p>
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<p>Return <a href="#top">to top of page</a> or
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<a href="../faq/index.html">to the FAQ</a>.
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</p>
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<!-- ####################################################### -->
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<hr>
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<p class="fineprint"><em>
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See <a href="../17_intro/license.html">license.html</a> for copying conditions.
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Comments and suggestions are welcome, and may be sent to
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<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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</em></p>
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</body>
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</html>
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