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<HTML><!-- -- Copyright (c) 1996-1999 -- Silicon Graphics Computer Systems, Inc. -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Silicon Graphics makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- -- Copyright (c) 1994 -- Hewlett-Packard Company -- -- Permission to use, copy, modify, distribute and sell this software -- and its documentation for any purpose is hereby granted without fee, -- provided that the above copyright notice appears in all copies and -- that both that copyright notice and this permission notice appear -- in supporting documentation. Hewlett-Packard Company makes no -- representations about the suitability of this software for any -- purpose. It is provided "as is" without express or implied warranty. -- --><Head><Title>Allocators</Title><!-- Generated by htmldoc --></HEAD><BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" ALINK="#ff0000"> <IMG SRC="CorpID.gif" ALT="SGI" HEIGHT="43" WIDTH="151"> <!--end header--><BR Clear><H1>Allocators</H1><Table CellPadding=0 CellSpacing=0 width=100%><TR><TD Align=left><Img src = "allocators.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD><TD Align=right><Img src = "overview.gif" Alt="" WIDTH = "194" HEIGHT = "38" ></TD></TR><TR><TD Align=left VAlign=top><b>Category</b>: allocators</TD><TD Align=right VAlign=top><b>Component type</b>: overview</TD></TR></Table><h3>Summary</h3>Allocators encapsulate allocation and deallocation of memory. Theyprovide a low-level interface that permits efficient allocation of many small objects; different allocator types represent differentschemes for memory management.<P>Note that allocators simply allocate and deallocate memory, as opposedto creating and destroying objects. The STL also includes severallow-level algorithms for manipulating uninitialized memory.<P>Note also that allocators do not attempt to encapsulate multiplememory models. The C++ language only defines a single memory model(the difference of two pointers, for example, is always <tt>ptrdiff_t</tt>),and this memory model is the only one that allocators support. <i>This is a major change from the definition of allocators in theoriginal STL.</i> <A href="#1">[1]</A><h3>Description</h3>The details of the allocator interface are still subject to change,and we do not guarantee that specific member functions will remain infuture versions. You should think of an allocator as a "black box".That is, you may select a container's memory allocation strategy byinstantiating the container template with a particular allocator <A href="#2">[2]</A>,but you should not make any assumptions about how the container actually uses the allocator. <P>The available allocators are as follows. In most cases you shouldn'thave to worry about the distinction: the default allocator, <tt>alloc</tt>,is usually the best choice.<Table border><TR><TD VAlign=top><tt>alloc</tt></TD><TD VAlign=top>The default allocator. It is thread-safe, and usually has the best performance characteristics.</TD></TR><TR><TD VAlign=top><tt>pthread_alloc</tt></TD><TD VAlign=top>A thread-safe allocator that uses a different memory pool for each thread; you can only use <tt>pthread_alloc</tt> if your operating system provides pthreads. <tt>Pthread_alloc</tt> is usually faster than <tt>alloc</tt>, especially on multiprocessor systems. It can, however, cause resource fragmentation: memory deallocated in one thread is not available for use by other threads.</TD></TR><TR><TD VAlign=top><tt>single_client_alloc</tt></TD><TD VAlign=top>A fast but thread-unsafe allocator. In programs that only have one thread, this allocator might be faster than <tt>alloc</tt>.</TD></TR><TR><TD VAlign=top><tt>malloc_alloc</tt></TD><TD VAlign=top>An allocator that simply uses the standard library function <tt>malloc</tt>. It is thread-safe but slow; the main reason why you might sometimes want to use it is to get more useful information from bounds-checking or leak-detection tools while you are debugging.</TD></tr></table><h3>Examples</h3><pre>vector<double> V(100, 5.0); // Uses the default allocator.vector<double, single_client_alloc> local(V.begin(), V.end());</pre><h3>Concepts</h3><UL><LI>Allocator</UL><h3>Types</h3><UL><LI><tt>alloc</tt><LI><tt>pthread_alloc</tt><LI><tt>single_client_alloc</tt><LI><tt>malloc_alloc</tt><LI><tt><A href="raw_storage_iterator.html">raw_storage_iterator</A></tt></UL><h3>Functions</h3><UL><LI><tt><A href="construct.html">construct</A></tt><LI><tt><A href="destroy.html">destroy</A></tt><LI><tt><A href="uninitialized_copy.html">uninitialized_copy</A></tt><LI><tt><A href="uninitialized_fill.html">uninitialized_fill</A></tt><LI><tt><A href="uninitialized_fill_n.html">uninitialized_fill_n</A></tt><LI><tt><A href="get_temporary_buffer.html">get_temporary_buffer</A></tt><LI><tt><A href="return_temporary_buffer.html">return_temporary_buffer</A></tt></UL><h3>Notes</h3><P><A name="1">[1]</A>The reason for this change is that the new interface reducesmemory fragmentation, and that it allows an implementation thatis both efficient and thread-safe.<P><A name="2">[2]</A>Different containers may use different allocators. You might,for example, have some containers that use the default allocator<tt>alloc</tt> and others that use <tt>pthread_alloc</tt>. Note, however, that<tt>vector<int></tt> and <tt>vector<int, pthread_alloc></tt> are distinct types.<h3>See also</h3><!--start footer--> <HR SIZE="6"><A href="http://www.sgi.com/"><IMG SRC="surf.gif" HEIGHT="54" WIDTH="54" ALT="[Silicon Surf]"></A><A HREF="index.html"><IMG SRC="stl_home.gif" HEIGHT="54" WIDTH="54" ALT="[STL Home]"></A><BR><FONT SIZE="-2"><A href="http://www.sgi.com/Misc/sgi_info.html" TARGET="_top">Copyright © 1999 Silicon Graphics, Inc.</A> All Rights Reserved.</FONT><FONT SIZE="-3"><a href="http://www.sgi.com/Misc/external.list.html" TARGET="_top">TrademarkInformation</A></FONT><P></BODY></HTML> ?? 快捷鍵說明
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