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
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<META NAME="AUTHOR" CONTENT="pme@sources.redhat.com (Phil Edwards)">
<META NAME="KEYWORDS" CONTENT="HOWTO, libstdc++, GCC, g++, libg++, STL">
<META NAME="DESCRIPTION" CONTENT="HOWTO for the libstdc++ chapter 24.">
<META NAME="GENERATOR" CONTENT="vi and eight fingers">
<TITLE>libstdc++-v3 HOWTO: Chapter 24</TITLE>
<LINK REL=StyleSheet HREF="../lib3styles.css">
<!-- $Id: howto.html,v 1.4 2000/10/21 00:51:50 jsm28 Exp $ -->
</HEAD>
<BODY>
<H1 CLASS="centered"><A NAME="top">Chapter 24: Iterators</A></H1>
<P>Chapter 24 deals with the FORTRAN subroutines for automatically
transforming lemmings into gold.
</P>
<!-- ####################################################### -->
<HR>
<H1>Contents</H1>
<UL>
<LI><A HREF="#1">They ain't pointers!</A>
<LI><A HREF="#2">It ends <EM>where?</EM></A>
</UL>
<HR>
<!-- ####################################################### -->
<H2><A NAME="1">They ain't pointers!</A></H2>
<P><A HREF="../faq/index.html#5_1">FAQ 5.1</A> points out that iterators
are not implemented as pointers. They are a generalization of
pointers, but they are implemented in libstdc++-v3 as separate classes.
</P>
<P>Keeping that simple fact in mind as you design your code will
prevent a whole lot of difficult-to-understand bugs.
</P>
<P>You can think of it the other way 'round, even. Since iterators
are a generalization, that means that <EM>pointers</EM> are
<EM>iterators</EM>, and that pointers can be used whenever an
iterator would be. All those functions in the Algorithms chapter
of the Standard will work just as well on plain arrays and their
pointers.
</P>
<P>That doesn't mean that when you pass in a pointer, it gets wrapped
into some special delegating iterator-to-pointer class with a layer
of overhead. (If you think that's the case anywhere, you don't
understand templates to begin with...) Oh, no; if you pass
in a pointer, then the compiler will instantiate that template
using T* as a type and good old high-speed pointer arithmetic as
its operations, so the resulting code will be doing exactly the same
things as it would be doing if you had hand-coded it yourself (for
the 273rd time).
</P>
<P>How much overhead <EM>is</EM> there when using an interator class?
Very little. Most of the layering classes contain nothing but
typedefs, and typedefs are "meta-information" that simply
tell the compiler some nicknames; they don't create code. That
information gets passed down through inheritance, so while the
compiler has to do work looking up all the names, your runtime code
does not. (This has been a prime concern from the beginning.)
</P>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
</P>
<HR>
<H2><A NAME="2">It ends <EM>where?</EM></A></H2>
<P>Blah.
</P>
<P>Return <A HREF="#top">to top of page</A> or
<A HREF="../faq/index.html">to the FAQ</A>.
</P>
<!-- ####################################################### -->
<HR>
<P CLASS="fineprint"><EM>
Comments and suggestions are welcome, and may be sent to
<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
<BR> $Id: howto.html,v 1.4 2000/10/21 00:51:50 jsm28 Exp $
</EM></P>
</BODY>
</HTML>
|
