<?xml version="1.0" encoding="UTF-8" standalone="no"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><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 canhappen with misuse of the <code class="classname">auto_ptr</code> classtemplate (called <acronym class="acronym">AP</acronym> here) would take sometime. 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 reallynifty idea for a smart pointer, but it is one of the dumbest ofall the smart pointers -- and that's fine.</p><p>AP is not meant to be a supersmart solution to all resourceleaks everywhere. Neither is it meant to be an effective formof 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 thepresence of exceptions. That's <span class="emphasis"><em>all</em></span>. Thiscode 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 exceptionfunction_taking_MyClass_pointer (ap.get());}</pre><p>When an exception gets thrown, the instance of MyClass that'sbeen created on the heap will be <code class="function">delete</code>'d as the stack isunwound 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 useof <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 pointsto one or many things. If it points to many things, you are aboutto die. AP is trivial to write, however, so you could write yourown <code class="code">auto_array_ptr</code> for that situation (in fact, this hasbeen 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 typesto 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 notmeet this requirement. Creating a new AP by copying an existingone transfers ownership of the pointed-to object, which means thatthe AP being copied must change, which in turn means that thecopy ctors of AP do not take const objects.</p><p>The resulting rule is simple: <span class="emphasis"><em>Never ever use acontainer of auto_ptr objects</em></span>. The standard says that“<span class="quote">undefined</span>” behavior is the result, but it isguaranteed 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> builtin to this implementation will issue an error if you try tocompile 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. 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