2 <!DOCTYPE part PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
6 <part id="manual.iterators" xreflabel="Iterators">
7 <?dbhtml filename="iterators.html"?>
20 <title>Iterators</title>
22 <!-- Chapter 01 : Predefined -->
23 <chapter id="manual.iterators.predefined" xreflabel="Predefined">
24 <title>Predefined</title>
26 <sect1 id="iterators.predefined.vs_pointers" xreflabel="Versus Pointers">
27 <title>Iterators vs. Pointers</title>
28 <para><ulink url="../faq/index.html#5_1">FAQ 5.1</ulink> points out that iterators
29 are not implemented as pointers. They are a generalization of
30 pointers, but they are implemented in libstdc++ as separate classes.
32 <para>Keeping that simple fact in mind as you design your code will
33 prevent a whole lot of difficult-to-understand bugs.
35 <para>You can think of it the other way 'round, even. Since iterators
36 are a generalization, that means that <emphasis>pointers</emphasis> are
37 <emphasis>iterators</emphasis>, and that pointers can be used whenever an
38 iterator would be. All those functions in the Algorithms chapter
39 of the Standard will work just as well on plain arrays and their
42 <para>That doesn't mean that when you pass in a pointer, it gets wrapped
43 into some special delegating iterator-to-pointer class with a layer
44 of overhead. (If you think that's the case anywhere, you don't
45 understand templates to begin with...) Oh, no; if you pass
46 in a pointer, then the compiler will instantiate that template
47 using T* as a type, and good old high-speed pointer arithmetic as
48 its operations, so the resulting code will be doing exactly the same
49 things as it would be doing if you had hand-coded it yourself (for
52 <para>How much overhead <emphasis>is</emphasis> there when using an iterator class?
53 Very little. Most of the layering classes contain nothing but
54 typedefs, and typedefs are "meta-information" that simply
55 tell the compiler some nicknames; they don't create code. That
56 information gets passed down through inheritance, so while the
57 compiler has to do work looking up all the names, your runtime code
58 does not. (This has been a prime concern from the beginning.)
64 <sect1 id="iterators.predefined.end" xreflabel="end() Is One Past the End">
65 <title>One Past the End</title>
67 <para>This starts off sounding complicated, but is actually very easy,
68 especially towards the end. Trust me.
70 <para>Beginners usually have a little trouble understand the whole
71 'past-the-end' thing, until they remember their early algebra classes
72 (see, they <emphasis>told</emphasis> you that stuff would come in handy!) and
73 the concept of half-open ranges.
75 <para>First, some history, and a reminder of some of the funkier rules in
76 C and C++ for builtin arrays. The following rules have always been
77 true for both languages:
81 <para>You can point anywhere in the array, <emphasis>or to the first element
82 past the end of the array</emphasis>. A pointer that points to one
83 past the end of the array is guaranteed to be as unique as a
84 pointer to somewhere inside the array, so that you can compare
89 <para>You can only dereference a pointer that points into an array.
90 If your array pointer points outside the array -- even to just
91 one past the end -- and you dereference it, Bad Things happen.
95 <para>Strictly speaking, simply pointing anywhere else invokes
96 undefined behavior. Most programs won't puke until such a
97 pointer is actually dereferenced, but the standards leave that
102 <para>The reason this past-the-end addressing was allowed is to make it
103 easy to write a loop to go over an entire array, e.g.,
104 while (*d++ = *s++);.
106 <para>So, when you think of two pointers delimiting an array, don't think
107 of them as indexing 0 through n-1. Think of them as <emphasis>boundary
114 | | This is bad. Always having to
115 | | remember to add or subtract one.
116 | | Off-by-one bugs very common here.
119 |---|---|--...--|---|---|
120 | 0 | 1 | ... |N-2|N-1|
121 |---|---|--...--|---|---|
125 | | This is good. This is safe. This
126 | | is guaranteed to work. Just don't
127 | | dereference 'end'.
131 <para>See? Everything between the boundary markers is part of the array.
134 <para>Now think back to your junior-high school algebra course, when you
135 were learning how to draw graphs. Remember that a graph terminating
136 with a solid dot meant, "Everything up through this point,"
137 and a graph terminating with an open dot meant, "Everything up
138 to, but not including, this point," respectively called closed
139 and open ranges? Remember how closed ranges were written with
140 brackets, <emphasis>[a,b]</emphasis>, and open ranges were written with parentheses,
141 <emphasis>(a,b)</emphasis>?
143 <para>The boundary markers for arrays describe a <emphasis>half-open range</emphasis>,
144 starting with (and including) the first element, and ending with (but
145 not including) the last element: <emphasis>[beginning,end)</emphasis>. See, I
146 told you it would be simple in the end.
148 <para>Iterators, and everything working with iterators, follows this same
149 time-honored tradition. A container's <code>begin()</code> method returns
150 an iterator referring to the first element, and its <code>end()</code>
151 method returns a past-the-end iterator, which is guaranteed to be
152 unique and comparable against any other iterator pointing into the
153 middle of the container.
155 <para>Container constructors, container methods, and algorithms, all take
156 pairs of iterators describing a range of values on which to operate.
157 All of these ranges are half-open ranges, so you pass the beginning
158 iterator as the starting parameter, and the one-past-the-end iterator
159 as the finishing parameter.
161 <para>This generalizes very well. You can operate on sub-ranges quite
162 easily this way; functions accepting a <emphasis>[first,last)</emphasis> range
163 don't know or care whether they are the boundaries of an entire {array,
164 sequence, container, whatever}, or whether they only enclose a few
165 elements from the center. This approach also makes zero-length
166 sequences very simple to recognize: if the two endpoints compare
167 equal, then the {array, sequence, container, whatever} is empty.
169 <para>Just don't dereference <code>end()</code>.
175 <!-- Chapter 02 : Stream -->