1 <chapter xmlns="http://docbook.org/ns/docbook" version="5.0" 2 xml:id="std.iterators" xreflabel="Iterators"> 3 <?dbhtml filename="iterators.html"?> 4 5 <info><title> 6 Iterators 7 <indexterm><primary>Iterators</primary></indexterm> 8 </title> 9 <keywordset> 10 <keyword>ISO C++</keyword> 11 <keyword>library</keyword> 12 </keywordset> 13 </info> 14 15 16 17 <!-- Sect1 01 : Predefined --> 18 <section xml:id="std.iterators.predefined" xreflabel="Predefined"><info><title>Predefined</title></info> 19 20 21 <section xml:id="iterators.predefined.vs_pointers" xreflabel="Versus Pointers"><info><title>Iterators vs. Pointers</title></info> 22 23 <para> 24 The following 25 FAQ <link linkend="faq.iterator_as_pod">entry</link> points out that 26 iterators are not implemented as pointers. They are a generalization 27 of pointers, but they are implemented in libstdc++ as separate 28 classes. 29 </para> 30 <para> 31 Keeping that simple fact in mind as you design your code will 32 prevent a whole lot of difficult-to-understand bugs. 33 </para> 34 <para> 35 You can think of it the other way 'round, even. Since iterators 36 are a generalization, that means 37 that <emphasis>pointers</emphasis> are 38 <emphasis>iterators</emphasis>, and that pointers can be used 39 whenever an iterator would be. All those functions in the 40 Algorithms section of the Standard will work just as well on plain 41 arrays and their pointers. 42 </para> 43 <para> 44 That doesn't mean that when you pass in a pointer, it gets 45 wrapped into some special delegating iterator-to-pointer class 46 with a layer of overhead. (If you think that's the case 47 anywhere, you don't understand templates to begin with...) Oh, 48 no; if you pass in a pointer, then the compiler will instantiate 49 that template using T* as a type, and good old high-speed 50 pointer arithmetic as its operations, so the resulting code will 51 be doing exactly the same things as it would be doing if you had 52 hand-coded it yourself (for the 273rd time). 53 </para> 54 <para> 55 How much overhead <emphasis>is</emphasis> there when using an 56 iterator class? Very little. Most of the layering classes 57 contain nothing but typedefs, and typedefs are 58 "meta-information" that simply tell the compiler some 59 nicknames; they don't create code. That information gets passed 60 down through inheritance, so while the compiler has to do work 61 looking up all the names, your runtime code does not. (This has 62 been a prime concern from the beginning.) 63 </para> 64 65 66 </section> 67 68 <section xml:id="iterators.predefined.end" xreflabel="end() Is One Past the End"><info><title>One Past the End</title></info> 69 70 71 <para>This starts off sounding complicated, but is actually very easy, 72 especially towards the end. Trust me. 73 </para> 74 <para>Beginners usually have a little trouble understand the whole 75 'past-the-end' thing, until they remember their early algebra classes 76 (see, they <emphasis>told</emphasis> you that stuff would come in handy!) and 77 the concept of half-open ranges. 78 </para> 79 <para>First, some history, and a reminder of some of the funkier rules in 80 C and C++ for builtin arrays. The following rules have always been 81 true for both languages: 82 </para> 83 <orderedlist inheritnum="ignore" continuation="restarts"> 84 <listitem> 85 <para>You can point anywhere in the array, <emphasis>or to the first element 86 past the end of the array</emphasis>. A pointer that points to one 87 past the end of the array is guaranteed to be as unique as a 88 pointer to somewhere inside the array, so that you can compare 89 such pointers safely. 90 </para> 91 </listitem> 92 <listitem> 93 <para>You can only dereference a pointer that points into an array. 94 If your array pointer points outside the array -- even to just 95 one past the end -- and you dereference it, Bad Things happen. 96 </para> 97 </listitem> 98 <listitem> 99 <para>Strictly speaking, simply pointing anywhere else invokes 100 undefined behavior. Most programs won't puke until such a 101 pointer is actually dereferenced, but the standards leave that 102 up to the platform. 103 </para> 104 </listitem> 105 </orderedlist> 106 <para>The reason this past-the-end addressing was allowed is to make it 107 easy to write a loop to go over an entire array, e.g., 108 while (*d++ = *s++);. 109 </para> 110 <para>So, when you think of two pointers delimiting an array, don't think 111 of them as indexing 0 through n-1. Think of them as <emphasis>boundary 112 markers</emphasis>: 113 </para> 114 <programlisting> 115 116 beginning end 117 | | 118 | | This is bad. Always having to 119 | | remember to add or subtract one. 120 | | Off-by-one bugs very common here. 121 V V 122 array of N elements 123 |---|---|--...--|---|---| 124 | 0 | 1 | ... |N-2|N-1| 125 |---|---|--...--|---|---| 126 127 ^ ^ 128 | | 129 | | This is good. This is safe. This 130 | | is guaranteed to work. Just don't 131 | | dereference 'end'. 132 beginning end 133 134 </programlisting> 135 <para>See? Everything between the boundary markers is chapter of the array. 136 Simple. 137 </para> 138 <para>Now think back to your junior-high school algebra course, when you 139 were learning how to draw graphs. Remember that a graph terminating 140 with a solid dot meant, "Everything up through this point," 141 and a graph terminating with an open dot meant, "Everything up 142 to, but not including, this point," respectively called closed 143 and open ranges? Remember how closed ranges were written with 144 brackets, <emphasis>[a,b]</emphasis>, and open ranges were written with parentheses, 145 <emphasis>(a,b)</emphasis>? 146 </para> 147 <para>The boundary markers for arrays describe a <emphasis>half-open range</emphasis>, 148 starting with (and including) the first element, and ending with (but 149 not including) the last element: <emphasis>[beginning,end)</emphasis>. See, I 150 told you it would be simple in the end. 151 </para> 152 <para>Iterators, and everything working with iterators, follows this same 153 time-honored tradition. A container's <code>begin()</code> method returns 154 an iterator referring to the first element, and its <code>end()</code> 155 method returns a past-the-end iterator, which is guaranteed to be 156 unique and comparable against any other iterator pointing into the 157 middle of the container. 158 </para> 159 <para>Container constructors, container methods, and algorithms, all take 160 pairs of iterators describing a range of values on which to operate. 161 All of these ranges are half-open ranges, so you pass the beginning 162 iterator as the starting parameter, and the one-past-the-end iterator 163 as the finishing parameter. 164 </para> 165 <para>This generalizes very well. You can operate on sub-ranges quite 166 easily this way; functions accepting a <emphasis>[first,last)</emphasis> range 167 don't know or care whether they are the boundaries of an entire {array, 168 sequence, container, whatever}, or whether they only enclose a few 169 elements from the center. This approach also makes zero-length 170 sequences very simple to recognize: if the two endpoints compare 171 equal, then the {array, sequence, container, whatever} is empty. 172 </para> 173 <para>Just don't dereference <code>end()</code>. 174 </para> 175 176 </section> 177 </section> 178 179 <!-- Sect1 02 : Stream --> 180 181 </chapter> 182
Indexes created Sun May 03 00:22:47 UTC 2026