1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
|
<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. They
provide a low-level interface that permits efficient allocation of
many small objects; different allocator types represent different
schemes for memory management.
<P>
Note that allocators simply allocate and deallocate memory, as opposed
to creating and destroying objects. The STL also includes several
low-level algorithms for manipulating uninitialized memory.
<P>
Note also that allocators do not attempt to encapsulate multiple
memory 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 the
original 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 in
future versions. You should think of an allocator as a "black box".
That is, you may select a container's memory allocation strategy by
instantiating 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't
have 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 reduces
memory fragmentation, and that it allows an implementation that
is 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>
|
