| 1 |
742 |
jeremybenn |
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
| 2 |
|
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
|
| 3 |
|
|
<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Chapter 10. Iterators</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="bk01pt02.html" title="Part II. Standard Contents"/><link rel="prev" href="containers_and_c.html" title="Interacting with C"/><link rel="next" href="algorithms.html" title="Chapter 11. Algorithms"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 10.
|
| 4 |
|
|
Iterators
|
| 5 |
|
|
|
| 6 |
|
|
</th></tr><tr><td align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><th width="60%" align="center">Part II.
|
| 7 |
|
|
Standard Contents
|
| 8 |
|
|
</th><td align="right"> <a accesskey="n" href="algorithms.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 10. Iterators"><div class="titlepage"><div><div><h2 class="title"><a id="std.iterators"/>Chapter 10.
|
| 9 |
|
|
Iterators
|
| 10 |
|
|
<a id="id504099" class="indexterm"/>
|
| 11 |
|
|
</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="section"><a href="iterators.html#std.iterators.predefined">Predefined</a></span></dt><dd><dl><dt><span class="section"><a href="iterators.html#iterators.predefined.vs_pointers">Iterators vs. Pointers</a></span></dt><dt><span class="section"><a href="iterators.html#iterators.predefined.end">One Past the End</a></span></dt></dl></dd></dl></div><div class="section" title="Predefined"><div class="titlepage"><div><div><h2 class="title"><a id="std.iterators.predefined"/>Predefined</h2></div></div></div><div class="section" title="Iterators vs. Pointers"><div class="titlepage"><div><div><h3 class="title"><a id="iterators.predefined.vs_pointers"/>Iterators vs. Pointers</h3></div></div></div><p>
|
| 12 |
|
|
The following
|
| 13 |
|
|
FAQ <a class="link" href="../faq.html#faq.iterator_as_pod" title="7.1.">entry</a> points out that
|
| 14 |
|
|
iterators are not implemented as pointers. They are a generalization
|
| 15 |
|
|
of pointers, but they are implemented in libstdc++ as separate
|
| 16 |
|
|
classes.
|
| 17 |
|
|
</p><p>
|
| 18 |
|
|
Keeping that simple fact in mind as you design your code will
|
| 19 |
|
|
prevent a whole lot of difficult-to-understand bugs.
|
| 20 |
|
|
</p><p>
|
| 21 |
|
|
You can think of it the other way 'round, even. Since iterators
|
| 22 |
|
|
are a generalization, that means
|
| 23 |
|
|
that <span class="emphasis"><em>pointers</em></span> are
|
| 24 |
|
|
<span class="emphasis"><em>iterators</em></span>, and that pointers can be used
|
| 25 |
|
|
whenever an iterator would be. All those functions in the
|
| 26 |
|
|
Algorithms section of the Standard will work just as well on plain
|
| 27 |
|
|
arrays and their pointers.
|
| 28 |
|
|
</p><p>
|
| 29 |
|
|
That doesn't mean that when you pass in a pointer, it gets
|
| 30 |
|
|
wrapped into some special delegating iterator-to-pointer class
|
| 31 |
|
|
with a layer of overhead. (If you think that's the case
|
| 32 |
|
|
anywhere, you don't understand templates to begin with...) Oh,
|
| 33 |
|
|
no; if you pass in a pointer, then the compiler will instantiate
|
| 34 |
|
|
that template using T* as a type, and good old high-speed
|
| 35 |
|
|
pointer arithmetic as its operations, so the resulting code will
|
| 36 |
|
|
be doing exactly the same things as it would be doing if you had
|
| 37 |
|
|
hand-coded it yourself (for the 273rd time).
|
| 38 |
|
|
</p><p>
|
| 39 |
|
|
How much overhead <span class="emphasis"><em>is</em></span> there when using an
|
| 40 |
|
|
iterator class? Very little. Most of the layering classes
|
| 41 |
|
|
contain nothing but typedefs, and typedefs are
|
| 42 |
|
|
"meta-information" that simply tell the compiler some
|
| 43 |
|
|
nicknames; they don't create code. That information gets passed
|
| 44 |
|
|
down through inheritance, so while the compiler has to do work
|
| 45 |
|
|
looking up all the names, your runtime code does not. (This has
|
| 46 |
|
|
been a prime concern from the beginning.)
|
| 47 |
|
|
</p></div><div class="section" title="One Past the End"><div class="titlepage"><div><div><h3 class="title"><a id="iterators.predefined.end"/>One Past the End</h3></div></div></div><p>This starts off sounding complicated, but is actually very easy,
|
| 48 |
|
|
especially towards the end. Trust me.
|
| 49 |
|
|
</p><p>Beginners usually have a little trouble understand the whole
|
| 50 |
|
|
'past-the-end' thing, until they remember their early algebra classes
|
| 51 |
|
|
(see, they <span class="emphasis"><em>told</em></span> you that stuff would come in handy!) and
|
| 52 |
|
|
the concept of half-open ranges.
|
| 53 |
|
|
</p><p>First, some history, and a reminder of some of the funkier rules in
|
| 54 |
|
|
C and C++ for builtin arrays. The following rules have always been
|
| 55 |
|
|
true for both languages:
|
| 56 |
|
|
</p><div class="orderedlist"><ol class="orderedlist"><li class="listitem"><p>You can point anywhere in the array, <span class="emphasis"><em>or to the first element
|
| 57 |
|
|
past the end of the array</em></span>. A pointer that points to one
|
| 58 |
|
|
past the end of the array is guaranteed to be as unique as a
|
| 59 |
|
|
pointer to somewhere inside the array, so that you can compare
|
| 60 |
|
|
such pointers safely.
|
| 61 |
|
|
</p></li><li class="listitem"><p>You can only dereference a pointer that points into an array.
|
| 62 |
|
|
If your array pointer points outside the array -- even to just
|
| 63 |
|
|
one past the end -- and you dereference it, Bad Things happen.
|
| 64 |
|
|
</p></li><li class="listitem"><p>Strictly speaking, simply pointing anywhere else invokes
|
| 65 |
|
|
undefined behavior. Most programs won't puke until such a
|
| 66 |
|
|
pointer is actually dereferenced, but the standards leave that
|
| 67 |
|
|
up to the platform.
|
| 68 |
|
|
</p></li></ol></div><p>The reason this past-the-end addressing was allowed is to make it
|
| 69 |
|
|
easy to write a loop to go over an entire array, e.g.,
|
| 70 |
|
|
while (*d++ = *s++);.
|
| 71 |
|
|
</p><p>So, when you think of two pointers delimiting an array, don't think
|
| 72 |
|
|
of them as indexing 0 through n-1. Think of them as <span class="emphasis"><em>boundary
|
| 73 |
|
|
markers</em></span>:
|
| 74 |
|
|
</p><pre class="programlisting">
|
| 75 |
|
|
|
| 76 |
|
|
beginning end
|
| 77 |
|
|
| |
|
| 78 |
|
|
| | This is bad. Always having to
|
| 79 |
|
|
| | remember to add or subtract one.
|
| 80 |
|
|
| | Off-by-one bugs very common here.
|
| 81 |
|
|
V V
|
| 82 |
|
|
array of N elements
|
| 83 |
|
|
|---|---|--...--|---|---|
|
| 84 |
|
|
| 0 | 1 | ... |N-2|N-1|
|
| 85 |
|
|
|---|---|--...--|---|---|
|
| 86 |
|
|
|
| 87 |
|
|
^ ^
|
| 88 |
|
|
| |
|
| 89 |
|
|
| | This is good. This is safe. This
|
| 90 |
|
|
| | is guaranteed to work. Just don't
|
| 91 |
|
|
| | dereference 'end'.
|
| 92 |
|
|
beginning end
|
| 93 |
|
|
|
| 94 |
|
|
</pre><p>See? Everything between the boundary markers is chapter of the array.
|
| 95 |
|
|
Simple.
|
| 96 |
|
|
</p><p>Now think back to your junior-high school algebra course, when you
|
| 97 |
|
|
were learning how to draw graphs. Remember that a graph terminating
|
| 98 |
|
|
with a solid dot meant, "Everything up through this point,"
|
| 99 |
|
|
and a graph terminating with an open dot meant, "Everything up
|
| 100 |
|
|
to, but not including, this point," respectively called closed
|
| 101 |
|
|
and open ranges? Remember how closed ranges were written with
|
| 102 |
|
|
brackets, <span class="emphasis"><em>[a,b]</em></span>, and open ranges were written with parentheses,
|
| 103 |
|
|
<span class="emphasis"><em>(a,b)</em></span>?
|
| 104 |
|
|
</p><p>The boundary markers for arrays describe a <span class="emphasis"><em>half-open range</em></span>,
|
| 105 |
|
|
starting with (and including) the first element, and ending with (but
|
| 106 |
|
|
not including) the last element: <span class="emphasis"><em>[beginning,end)</em></span>. See, I
|
| 107 |
|
|
told you it would be simple in the end.
|
| 108 |
|
|
</p><p>Iterators, and everything working with iterators, follows this same
|
| 109 |
|
|
time-honored tradition. A container's <code class="code">begin()</code> method returns
|
| 110 |
|
|
an iterator referring to the first element, and its <code class="code">end()</code>
|
| 111 |
|
|
method returns a past-the-end iterator, which is guaranteed to be
|
| 112 |
|
|
unique and comparable against any other iterator pointing into the
|
| 113 |
|
|
middle of the container.
|
| 114 |
|
|
</p><p>Container constructors, container methods, and algorithms, all take
|
| 115 |
|
|
pairs of iterators describing a range of values on which to operate.
|
| 116 |
|
|
All of these ranges are half-open ranges, so you pass the beginning
|
| 117 |
|
|
iterator as the starting parameter, and the one-past-the-end iterator
|
| 118 |
|
|
as the finishing parameter.
|
| 119 |
|
|
</p><p>This generalizes very well. You can operate on sub-ranges quite
|
| 120 |
|
|
easily this way; functions accepting a <span class="emphasis"><em>[first,last)</em></span> range
|
| 121 |
|
|
don't know or care whether they are the boundaries of an entire {array,
|
| 122 |
|
|
sequence, container, whatever}, or whether they only enclose a few
|
| 123 |
|
|
elements from the center. This approach also makes zero-length
|
| 124 |
|
|
sequences very simple to recognize: if the two endpoints compare
|
| 125 |
|
|
equal, then the {array, sequence, container, whatever} is empty.
|
| 126 |
|
|
</p><p>Just don't dereference <code class="code">end()</code>.
|
| 127 |
|
|
</p></div></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><td align="center"><a accesskey="u" href="bk01pt02.html">Up</a></td><td align="right"> <a accesskey="n" href="algorithms.html">Next</a></td></tr><tr><td align="left" valign="top">Interacting with C </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Chapter 11.
|
| 128 |
|
|
Algorithms
|
| 129 |
|
|
|
| 130 |
|
|
</td></tr></table></div></body></html>
|
