0075846f90
2009-03-17 Benjamin Kosnik <bkoz@redhat.com> * doc/xml/manual/appendix_contributing.xml: Add docbook style sheet version information. Table-ize docbook element examples. * doc/xml/manual/using.xml: Human-readable header markup. Alphabetized. Add new headers. * doc/html: Regenerate. From-SVN: r144923
90 lines
6.3 KiB
HTML
90 lines
6.3 KiB
HTML
<?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="bk01pt04ch11.html" title="Chapter 11. Memory" /><link rel="prev" href="bk01pt04ch11.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="bk01pt04ch11.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="bk01pt04ch11.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt04ch11.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>
|