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 <A HREF="15-2.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="16.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library User's Guide</B></DIV>
 <H2>15.3 Building Your Own Allocators</H2>
 <A NAME="idx377"><!></A>
 <P>Defining your own <B><I><A HREF="../stdlibref/allocator.html">allocator</A></I></B> is a relatively simple process. The C++ Standard Library describes a particular interface, consisting of types and functions. An allocator that conforms to the standard must match the syntactic requirements for these member functions and types. The C++ Standard Library also specifies a portion of the semantics for the allocator type.</P>
 <A NAME="1531"><H3>15.3.1 Using the Standard Allocator Interface</H3></A>
 <A NAME="idx378"><!></A>
 <P>An <B><I><A HREF="../stdlibref/allocator.html">allocator</A></I></B> that conforms to the C++ Standard Library allocator specification must have the following interface. The example uses <SAMP>my_allocator</SAMP> as a place holder for your own allocator name:</P>

 <UL><PRE>
 template &lt;class T&gt;
 class my_allocator
 {
   typedef <I>unsigned integral type</I>    size_type;
   typedef <I>signed integral type</I>      difference_type;
   typedef <I>pointer to T</I>              pointer;
   typedef <I>const pointer to T</I>        const_pointer;
   typedef T&amp;                        reference;
   typedef const T&amp;                  const_reference;
   typedef T                         value_type;

   template &lt;class U&gt;
   struct rebind { typedef allocator&lt;U&gt; other; };
 };
 </PRE></UL>
 <P>The rebind member allows a container to construct an allocator for some arbitrary type out of the allocator type provided as a template parameter. For instance, the <B><I><A HREF="../stdlibref/list.html">list</A></I></B> container gets an <B><I><A HREF="../stdlibref/allocator.html">allocator&lt;T&gt;</A></I></B> by default, but a <B><I>list</I></B> may well need to allocate <B><I>list_node</I></B>s as well as <B><I>T</I></B>'s. The container can construct an allocator for <B><I>list_node</I></B>s out of the allocator for <B><I>T</I></B>, which is the template parameter <B><I><A HREF="../stdlibref/allocator.html">Allocator</A></I></B> in this case, as follows:</P>

 <UL><PRE>
 Allocator::rebind&lt;list_node&gt;::other list_node_allocator;
 </PRE></UL>
 <A NAME="idx379"><!></A>
 <P>Here is a description of the member functions that an <B><I><A HREF="../stdlibref/allocator.html">allocator</A></I></B> class template must provide:</P>

 <A NAME="idx380"></A><PRE>
 <B>my_allocator</B>();
 template &lt;class U&gt;</PRE>
 <UL>
 </UL>


 <A NAME="idx381"></A><PRE><B>my_allocator</B>(const my_allocator&lt;U&gt;&amp;);
 template &lt;class U&gt;</PRE>
 <UL>
 <P>Constructors.</P>
 </UL>


 <A NAME="idx382"></A><PRE><B>~my_allocator</B>();</PRE>
 <UL>
 <P>Destructor.</P>
 </UL>


 <A NAME="idx383"></A><PRE><B>operator=</B>(const my_allocator&lt;U&gt;&amp;);</PRE>
 <UL>
 <P>Assignment operator.</P>
 </UL>


 <A NAME="idx384"></A><PRE><B>pointer address</B>(reference r) const;</PRE>
 <UL>
 <P>Returns the address of <SAMP>r</SAMP> as a <SAMP>pointer</SAMP> type. This function and the following function are used to convert references to pointers.</P>
 </UL>


 <A NAME="idx385"></A><PRE><B>const_pointer address</B>(const_reference r) const;</PRE>
 <UL>
 <P>Returns the address of <SAMP>r</SAMP> as a <SAMP>const_pointer</SAMP> type.</P>
 </UL>


 <A NAME="idx386"></A><PRE><B>pointer allocate</B>(size_type n, allocator&lt;U&gt;::const_pointer hint=0);</PRE>
 <UL>
 <P>Allocates storage for <SAMP>n</SAMP> values of<SAMP> T</SAMP>. Uses the value of <SAMP>hint</SAMP> to optimize storage placement, if possible.</P>
 </UL>


 <A NAME="idx387"></A><PRE>void
 <B>deallocate</B>(pointer);</PRE>
 <UL>
 <P>Deallocates storage obtained by a call to <SAMP>allocate</SAMP>.</P>
 </UL>


 <A NAME="idx388"></A><PRE>size_type
 <B>max_size</B>();</PRE>
 <UL>
 <P>Returns the largest possible storage available through a call to <SAMP>allocate</SAMP>.</P>
 </UL>


 <A NAME="idx389"></A><PRE>void
 <B>construct</B>(pointer p, const_reference val);</PRE>
 <UL>
 <P>Constructs an object of type <SAMP>T</SAMP> at the location of <SAMP>p</SAMP>, using the value of <SAMP>val</SAMP> in the call to the constructor for <SAMP>T</SAMP>.</P>
 </UL>


 <A NAME="idx390"></A><PRE>void
 <B>destroy</B>(pointer p);</PRE>
 <UL>
 <P>Calls the destructor on the value pointed to by <SAMP>p</SAMP>.</P>
 </UL>

 <P>Additionally, user-defined allocators must be equality comparable: if <SAMP>a</SAMP> and <SAMP>b</SAMP> are instances of a user-defined allocator, then the expressions <SAMP>(a == b)</SAMP> and <SAMP>(a != b)</SAMP> must be well-formed.</P>

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	except in compliance with the License. You may obtain a copy of
	the License at

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	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
	implied. See the License for the specific language governing
	permissions and limitations under the License.

	Copyright 1999-2007 Rogue Wave Software, Inc.
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	<HTML>
	<HEAD>
	<TITLE>Building Your Own Allocators</TITLE>
	<LINK REL=StyleSheet HREF="../rw.css" TYPE="text/css" TITLE="Apache stdcxx Stylesheet"></HEAD>
	<BODY BGCOLOR=#FFFFFF>
	<A HREF="15-2.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="16.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library User's Guide</B></DIV>
	<H2>15.3 Building Your Own Allocators</H2>
	<A NAME="idx377"><!></A>
	<P>Defining your own <B><I><A HREF="../stdlibref/allocator.html">allocator</A></I></B> is a relatively simple process. The C++ Standard Library describes a particular interface, consisting of types and functions. An allocator that conforms to the standard must match the syntactic requirements for these member functions and types. The C++ Standard Library also specifies a portion of the semantics for the allocator type.</P>
	<A NAME="1531"><H3>15.3.1 Using the Standard Allocator Interface</H3></A>
	<A NAME="idx378"><!></A>
	<P>An <B><I><A HREF="../stdlibref/allocator.html">allocator</A></I></B> that conforms to the C++ Standard Library allocator specification must have the following interface. The example uses <SAMP>my_allocator</SAMP> as a place holder for your own allocator name:</P>

	<UL><PRE>
	template <class T>
	class my_allocator
	{
	typedef <I>unsigned integral type</I> size_type;
	typedef <I>signed integral type</I> difference_type;
	typedef <I>pointer to T</I> pointer;
	typedef <I>const pointer to T</I> const_pointer;
	typedef T& reference;
	typedef const T& const_reference;
	typedef T value_type;

	template <class U>
	struct rebind { typedef allocator<U> other; };
	};
	</PRE></UL>
	<P>The rebind member allows a container to construct an allocator for some arbitrary type out of the allocator type provided as a template parameter. For instance, the <B><I><A HREF="../stdlibref/list.html">list</A></I></B> container gets an <B><I><A HREF="../stdlibref/allocator.html">allocator<T></A></I></B> by default, but a <B><I>list</I></B> may well need to allocate <B><I>list_node</I></B>s as well as <B><I>T</I></B>'s. The container can construct an allocator for <B><I>list_node</I></B>s out of the allocator for <B><I>T</I></B>, which is the template parameter <B><I><A HREF="../stdlibref/allocator.html">Allocator</A></I></B> in this case, as follows:</P>

	<UL><PRE>
	Allocator::rebind<list_node>::other list_node_allocator;
	</PRE></UL>
	<A NAME="idx379"><!></A>
	<P>Here is a description of the member functions that an <B><I><A HREF="../stdlibref/allocator.html">allocator</A></I></B> class template must provide:</P>

	<A NAME="idx380"></A><PRE>
	<B>my_allocator</B>();
	template <class U></PRE>
	<UL>
	</UL>


	<A NAME="idx381"></A><PRE><B>my_allocator</B>(const my_allocator<U>&);
	template <class U></PRE>
	<UL>
	<P>Constructors.</P>
	</UL>


	<A NAME="idx382"></A><PRE><B>~my_allocator</B>();</PRE>
	<UL>
	<P>Destructor.</P>
	</UL>


	<A NAME="idx383"></A><PRE><B>operator=</B>(const my_allocator<U>&);</PRE>
	<UL>
	<P>Assignment operator.</P>
	</UL>


	<A NAME="idx384"></A><PRE><B>pointer address</B>(reference r) const;</PRE>
	<UL>
	<P>Returns the address of <SAMP>r</SAMP> as a <SAMP>pointer</SAMP> type. This function and the following function are used to convert references to pointers.</P>
	</UL>


	<A NAME="idx385"></A><PRE><B>const_pointer address</B>(const_reference r) const;</PRE>
	<UL>
	<P>Returns the address of <SAMP>r</SAMP> as a <SAMP>const_pointer</SAMP> type.</P>
	</UL>


	<A NAME="idx386"></A><PRE><B>pointer allocate</B>(size_type n, allocator<U>::const_pointer hint=0);</PRE>
	<UL>
	<P>Allocates storage for <SAMP>n</SAMP> values of<SAMP> T</SAMP>. Uses the value of <SAMP>hint</SAMP> to optimize storage placement, if possible.</P>
	</UL>


	<A NAME="idx387"></A><PRE>void
	<B>deallocate</B>(pointer);</PRE>
	<UL>
	<P>Deallocates storage obtained by a call to <SAMP>allocate</SAMP>.</P>
	</UL>


	<A NAME="idx388"></A><PRE>size_type
	<B>max_size</B>();</PRE>
	<UL>
	<P>Returns the largest possible storage available through a call to <SAMP>allocate</SAMP>.</P>
	</UL>


	<A NAME="idx389"></A><PRE>void
	<B>construct</B>(pointer p, const_reference val);</PRE>
	<UL>
	<P>Constructs an object of type <SAMP>T</SAMP> at the location of <SAMP>p</SAMP>, using the value of <SAMP>val</SAMP> in the call to the constructor for <SAMP>T</SAMP>.</P>
	</UL>


	<A NAME="idx390"></A><PRE>void
	<B>destroy</B>(pointer p);</PRE>
	<UL>
	<P>Calls the destructor on the value pointed to by <SAMP>p</SAMP>.</P>
	</UL>

	<P>Additionally, user-defined allocators must be equality comparable: if <SAMP>a</SAMP> and <SAMP>b</SAMP> are instances of a user-defined allocator, then the expressions <SAMP>(a == b)</SAMP> and <SAMP>(a != b)</SAMP> must be well-formed.</P>

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