libstdc++-v3/doc/xml/manual/auto_ptr.xml

<section xmlns="http://docbook.org/ns/docbook" version="5.0"
	 xml:id="std.util.memory.auto_ptr" xreflabel="auto_ptr">
<?dbhtml filename="auto_ptr.html"?>

<info><title>auto_ptr</title>
  <keywordset>
    <keyword>ISO C++</keyword>
    <keyword>auto_ptr</keyword>
  </keywordset>
</info>



<section xml:id="auto_ptr.limitations"><info><title>Limitations</title></info>


   <para>Explaining all of the fun and delicious things that can
   happen with misuse of the <classname>auto_ptr</classname> class
   template (called <acronym>AP</acronym> here) would take some
   time. Suffice it to say that the use of <acronym>AP</acronym>
   safely in the presence of copying has some subtleties.
   </para>
   <para>
     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.
   </para>
   <para>
     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 <emphasis>not</emphasis>be used for arrays!
   </para>
   <para>
     <acronym>AP</acronym> is meant to prevent nasty leaks in the
     presence of exceptions.  That's <emphasis>all</emphasis>.  This
     code is AP-friendly:
   </para>
<programlisting>
    // 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());
    }
</programlisting>
   <para>When an exception gets thrown, the instance of MyClass that's
      been created on the heap will be <function>delete</function>'d as the stack is
      unwound past <function>func()</function>.
   </para>
   <para>Changing that code as follows is not <acronym>AP</acronym>-friendly:
   </para>
<programlisting>
	APMC  ap (new MyClass[22]);
</programlisting>
   <para>You will get the same problems as you would without the use
      of <acronym>AP</acronym>:
   </para>
<programlisting>
	char*  array = new char[10];       // array new...
	...
	delete array;                      // ...but single-object delete
</programlisting>
   <para>
     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).
   </para>
</section>

<section xml:id="auto_ptr.using"><info><title>Use in Containers</title></info>


  <para>
  </para>
  <para>All of the <link linkend="std.containers">containers</link>
      described in the standard library require their contained types
      to have, among other things, a copy constructor like this:
  </para>
<programlisting>
    struct My_Type
    {
	My_Type (My_Type const&amp;);
    };
</programlisting>
   <para>
     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.
   </para>
   <para>
     The resulting rule is simple: <emphasis>Never ever use a
     container of auto_ptr objects</emphasis>. The standard says that
     <quote>undefined</quote> behavior is the result, but it is
     guaranteed to be messy.
   </para>
   <para>
     To prevent you from doing this to yourself, the
      <link linkend="manual.ext.compile_checks">concept checks</link> built
      in to this implementation will issue an error if you try to
      compile code like this:
   </para>
<programlisting>
    #include &lt;vector&gt;
    #include &lt;memory&gt;

    void f()
    {
	std::vector&lt; std::auto_ptr&lt;int&gt; &gt;   vec_ap_int;
    }
</programlisting>
   <para>
Should you try this with the checks enabled, you will see an error.
   </para>
</section>

</section>