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| <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>auto_ptr</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , auto_ptr " /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="memory.html" title="Chapter 11. Memory" /><link rel="prev" href="memory.html" title="Chapter 11. Memory" /><link rel="next" href="shared_ptr.html" title="shared_ptr" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">auto_ptr</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="memory.html">Prev</a> </td><th width="60%" align="center">Chapter 11. Memory</th><td width="20%" align="right"> <a accesskey="n" href="shared_ptr.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.util.memory.auto_ptr"></a>auto_ptr</h2></div></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="auto_ptr.limitations"></a>Limitations</h3></div></div></div><p>Explaining all of the fun and delicious things that can |
| happen with misuse of the <code class="classname">auto_ptr</code> class |
| template (called <acronym class="acronym">AP</acronym> here) would take some |
| time. Suffice it to say that the use of <acronym class="acronym">AP</acronym> |
| safely in the presence of copying has some subtleties. |
| </p><p> |
| 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 <span class="emphasis"><em>not</em></span>be used for arrays! |
| </p><p> |
| <acronym class="acronym">AP</acronym> is meant to prevent nasty leaks in the |
| presence of exceptions. That's <span class="emphasis"><em>all</em></span>. This |
| code is AP-friendly: |
| </p><pre class="programlisting"> |
| // Not a recommend naming scheme, but good for web-based FAQs. |
| typedef std::auto_ptr<MyClass> 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 class="function">delete</code>'d as the stack is |
| unwound past <code class="function">func()</code>. |
| </p><p>Changing that code as follows is not <acronym class="acronym">AP</acronym>-friendly: |
| </p><pre class="programlisting"> |
| APMC ap (new MyClass[22]); |
| </pre><p>You will get the same problems as you would without the use |
| of <acronym class="acronym">AP</acronym>: |
| </p><pre class="programlisting"> |
| 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 class="code">auto_array_ptr</code> for that situation (in fact, this has |
| been done many times; check the mailing lists, Usenet, Boost, etc). |
| </p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="auto_ptr.using"></a>Use in Containers</h3></div></div></div><p> |
| </p><p>All of the <a class="ulink" href="../23_containers/howto.html" target="_top">containers</a> |
| described in the standard library require their contained types |
| to have, among other things, a copy constructor like this: |
| </p><pre class="programlisting"> |
| struct My_Type |
| { |
| My_Type (My_Type const&); |
| }; |
| </pre><p> |
| Note the const keyword; the object being copied shouldn't change. |
| The template class <code 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: <span class="emphasis"><em>Never ever use a |
| container of auto_ptr objects</em></span>. The standard says that |
| “<span class="quote">undefined</span>” behavior is the result, but it is |
| guaranteed to be messy. |
| </p><p> |
| To prevent you from doing this to yourself, the |
| <a class="ulink" href="../19_diagnostics/howto.html#3" target="_top">concept checks</a> built |
| in to this implementation will issue an error if you try to |
| compile code like this: |
| </p><pre class="programlisting"> |
| #include <vector> |
| #include <memory> |
| |
| void f() |
| { |
| std::vector< std::auto_ptr<int> > vec_ap_int; |
| } |
| </pre><p> |
| Should you try this with the checks enabled, you will see an error. |
| </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="memory.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="memory.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="shared_ptr.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 11. Memory </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> shared_ptr</td></tr></table></div></body></html> |
