gcc/libstdc++-v3/docs/html/20_util/howto.html

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   <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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   <title>libstdc++-v3 HOWTO:  Chapter 20</title>
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<h1 class="centered"><a name="top">Chapter 20:  General Utilities</a></h1>

<p>Chapter 20 deals with utility classes and functions, such as
   the oft-debated <code>auto_ptr&lt;&gt;</code>.
</p>


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<h1>Contents</h1>
<ul>
   <li><a href="#1"><code>auto_ptr</code> is not omnipotent</a></li>
   <li><a href="#2"><code>auto_ptr</code> inside container classes</a></li>
   <li><a href="#3">Functors</a></li>
   <li><a href="#4">Pairs</a></li>
</ul>

<hr />

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<h2><a name="1"><code>auto_ptr</code> is not omnipotent</a></h2>
   <p>I'm not going to try and explain all of the fun and delicious
      things that can happen with misuse of the auto_ptr class template
      (called AP here), nor am I going to try and teach you how to use
      AP safely in the presence of copying.  The AP class is a really
      nifty idea for a smart pointer, but it is one of the dumbest of
      all the smart pointers -- and that's fine.
   </p>
   <p>AP is not meant to be a supersmart solution to all resource
      leaks everywhere.  Neither is it meant to be an effective form
      of garbage collection (although it can help, a little bit).
      And it can <em>not</em> be used for arrays!
   </p>
   <p>AP <em>is</em> meant to prevent nasty leaks in the presence of
      exceptions.  That's <em>all</em>.  This code is AP-friendly:
   </p>
   <pre>
    // not a recommend naming scheme, but good for web-based FAQs
    typedef std::auto_ptr&lt;MyClass&gt;  APMC;

    extern function_taking_MyClass_pointer (MyClass*);
    extern some_throwable_function ();

    void func (int data)
    {
        APMC  ap (new MyClass(data));

        some_throwable_function();   // this will throw an exception

        function_taking_MyClass_pointer (ap.get());
    }
   </pre>
   <p>When an exception gets thrown, the instance of MyClass that's
      been created on the heap will be <code>delete</code>'d as the stack is
      unwound past <code>func()</code>.
   </p>
   <p>Changing that code as follows is <em>not</em> AP-friendly:
   </p>
   <pre>
        APMC  ap (new MyClass[22]);
   </pre>
   <p>You will get the same problems as you would without the use
      of AP:
   </p>
   <pre>
        char*  array = new char[10];       // array new...
        ...
        delete array;                      // ...but single-object delete
   </pre>
   <p>AP cannot tell whether the pointer you've passed at creation points
      to one or many things.  If it points to many things, you are about
      to die.  AP is trivial to write, however, so you could write your
      own <code>auto_array_ptr</code> for that situation (in fact, this has
      been done many times; check the mailing lists, Usenet, Boost, etc).
   </p>
   <p>Return <a href="#top">to top of page</a> or
      <a href="../faq/index.html">to the FAQ</a>.
   </p>

<hr />
<h2><a name="2"><code>auto_ptr</code> inside container classes</a></h2>
   <p>All of the <a href="../23_containers/howto.html">containers</a>
      described in the standard library require their contained types
      to have, among other things, a copy constructor like this:
   </p>
   <pre>
    struct My_Type
    {
        My_Type (My_Type const&amp;);
    };
   </pre>
   <p>Note the const keyword; the object being copied shouldn't change.
      The template class <code>auto_ptr</code> (called AP here) does not
      meet this requirement.  Creating a new AP by copying an existing
      one transfers ownership of the pointed-to object, which means that
      the AP being copied must change, which in turn means that the
      copy ctors of AP do not take const objects.
   </p>
   <p>The resulting rule is simple:  <em>Never ever use a container of
      auto_ptr objects.</em>  The standard says that &quot;undefined&quot;
      behavior is the result, but it is guaranteed to be messy.
   </p>
   <p>To prevent you from doing this to yourself, the
      <a href="../19_diagnostics/howto.html#3">concept checks</a> built
      in to this implementation will issue an error if you try to
      compile code like this:
   </p>
   <pre>
    #include &lt;vector&gt;
    #include &lt;memory&gt;
    
    void f()
    {
        std::vector&lt; std::auto_ptr&lt;int&gt; &gt;   vec_ap_int;
    }
   </pre>
   <p>Should you try this with the checks enabled, you will see an error.
   </p>
   <p>Return <a href="#top">to top of page</a> or
      <a href="../faq/index.html">to the FAQ</a>.
   </p>

<hr />
<h2><a name="3">Functors</a></h2>
   <p>If you don't know what functors are, you're not alone.  Many people
      get slightly the wrong idea.  In the interest of not reinventing
      the wheel, we will refer you to the introduction to the functor
      concept written by SGI as part of their STL, in
      <a href="http://www.sgi.com/tech/stl/functors.html">their
      http://www.sgi.com/tech/stl/functors.html</a>.
   </p>
   <p>Return <a href="#top">to top of page</a> or
      <a href="../faq/index.html">to the FAQ</a>.
   </p>

<hr />
<h2><a name="4">Pairs</a></h2>
   <p>The <code>pair&lt;T1,T2&gt;</code> is a simple and handy way to
      carry around a pair of objects.  One is of type T1, and another of
      type T2; they may be the same type, but you don't get anything
      extra if they are.  The two members can be accessed directly, as
      <code>.first</code> and <code>.second</code>.
   </p>
   <p>Construction is simple.  The default ctor initializes each member
      with its respective default ctor.  The other simple ctor,
   </p>
   <pre>
    pair (const T1&amp; x, const T2&amp; y);
   </pre>
   <p>does what you think it does, <code>first</code> getting <code>x</code>
      and <code>second</code> getting <code>y</code>.
   </p>
   <p>There is a copy constructor, but it requires that your compiler
      handle member function templates:
   </p>
   <pre>
    template &lt;class U, class V&gt; pain (const pair&lt;U,V&gt;&amp; p);
   </pre>
   <p>The compiler will convert as necessary from U to T1 and from
      V to T2 in order to perform the respective initializations.
   </p>
   <p>The comparison operators are done for you.  Equality
      of two <code>pair&lt;T1,T2&gt;</code>s is defined as both <code>first</code>
      members comparing equal and both <code>second</code> members comparing
      equal; this simply delegates responsibility to the respective
      <code>operator==</code> functions (for types like MyClass) or builtin
      comparisons (for types like int, char, etc).
   </p>
   <p><a name="pairlt">
      The less-than operator is a bit odd the first time you see it.  It
      is defined as evaluating to:
      </a>
   </p>
   <pre>
    x.first  &lt;  y.first  ||
        ( !(y.first  &lt;  x.first)  &amp;&amp;  x.second  &lt;  y.second )
   </pre>
   <p>The other operators are not defined using the <code>rel_ops</code>
      functions above, but their semantics are the same.
   </p>
   <p>Finally, there is a template function called <code>make_pair</code>
      that takes two references-to-const objects and returns an
      instance of a pair instantiated on their respective types:
   </p>
   <pre>
    pair&lt;int,MyClass&gt; p = make_pair(4,myobject);
   </pre>
   <p>Return <a href="#top">to top of page</a> or
      <a href="../faq/index.html">to the FAQ</a>.
   </p>


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