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+<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="&#10; ISO C++&#10; , &#10; library&#10; "/><link rel="home" href="../spine.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. 
+ Iterators
+
+</th></tr><tr><td align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><th width="60%" align="center">Part II. 
+ Standard Contents
+ </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. 
+ Iterators
+ <a id="id479637" class="indexterm"/>
+</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>
+ The following
+FAQ <a class="link" href="../faq.html#faq.iterator_as_pod" title="7.1.">entry</a> points out that
+iterators are not implemented as pointers. They are a generalization
+of pointers, but they are implemented in libstdc++ 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 <span class="emphasis"><em>pointers</em></span> are
+ <span class="emphasis"><em>iterators</em></span>, and that pointers can be used
+ whenever an iterator would be. All those functions in the
+ Algorithms sect1 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 <span class="emphasis"><em>is</em></span> there when using an
+ iterator 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></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,
+ especially towards the end. Trust me.
+ </p><p>Beginners usually have a little trouble understand the whole
+ 'past-the-end' thing, until they remember their early algebra classes
+ (see, they <span class="emphasis"><em>told</em></span> you that stuff would come in handy!) and
+ the concept of half-open ranges.
+ </p><p>First, some history, and a reminder of some of the funkier rules in
+ C and C++ for builtin arrays. The following rules have always been
+ true for both languages:
+ </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
+ past the end of the array</em></span>. A pointer that points to one
+ past the end of the array is guaranteed to be as unique as a
+ pointer to somewhere inside the array, so that you can compare
+ such pointers safely.
+ </p></li><li class="listitem"><p>You can only dereference a pointer that points into an array.
+ If your array pointer points outside the array -- even to just
+ one past the end -- and you dereference it, Bad Things happen.
+ </p></li><li class="listitem"><p>Strictly speaking, simply pointing anywhere else invokes
+ undefined behavior. Most programs won't puke until such a
+ pointer is actually dereferenced, but the standards leave that
+ up to the platform.
+ </p></li></ol></div><p>The reason this past-the-end addressing was allowed is to make it
+ easy to write a loop to go over an entire array, e.g.,
+ while (*d++ = *s++);.
+ </p><p>So, when you think of two pointers delimiting an array, don't think
+ of them as indexing 0 through n-1. Think of them as <span class="emphasis"><em>boundary
+ markers</em></span>:
+ </p><pre class="programlisting">
+
+ beginning end
+ | |
+ | | This is bad. Always having to
+ | | remember to add or subtract one.
+ | | Off-by-one bugs very common here.
+ V V
+ array of N elements
+ |---|---|--...--|---|---|
+ | 0 | 1 | ... |N-2|N-1|
+ |---|---|--...--|---|---|
+
+ ^ ^
+ | |
+ | | This is good. This is safe. This
+ | | is guaranteed to work. Just don't
+ | | dereference 'end'.
+ beginning end
+
+ </pre><p>See? Everything between the boundary markers is chapter of the array.
+ Simple.
+ </p><p>Now think back to your junior-high school algebra course, when you
+ were learning how to draw graphs. Remember that a graph terminating
+ with a solid dot meant, "Everything up through this point,"
+ and a graph terminating with an open dot meant, "Everything up
+ to, but not including, this point," respectively called closed
+ and open ranges? Remember how closed ranges were written with
+ brackets, <span class="emphasis"><em>[a,b]</em></span>, and open ranges were written with parentheses,
+ <span class="emphasis"><em>(a,b)</em></span>?
+ </p><p>The boundary markers for arrays describe a <span class="emphasis"><em>half-open range</em></span>,
+ starting with (and including) the first element, and ending with (but
+ not including) the last element: <span class="emphasis"><em>[beginning,end)</em></span>. See, I
+ told you it would be simple in the end.
+ </p><p>Iterators, and everything working with iterators, follows this same
+ time-honored tradition. A container's <code class="code">begin()</code> method returns
+ an iterator referring to the first element, and its <code class="code">end()</code>
+ method returns a past-the-end iterator, which is guaranteed to be
+ unique and comparable against any other iterator pointing into the
+ middle of the container.
+ </p><p>Container constructors, container methods, and algorithms, all take
+ pairs of iterators describing a range of values on which to operate.
+ All of these ranges are half-open ranges, so you pass the beginning
+ iterator as the starting parameter, and the one-past-the-end iterator
+ as the finishing parameter.
+ </p><p>This generalizes very well. You can operate on sub-ranges quite
+ easily this way; functions accepting a <span class="emphasis"><em>[first,last)</em></span> range
+ don't know or care whether they are the boundaries of an entire {array,
+ sequence, container, whatever}, or whether they only enclose a few
+ elements from the center. This approach also makes zero-length
+ sequences very simple to recognize: if the two endpoints compare
+ equal, then the {array, sequence, container, whatever} is empty.
+ </p><p>Just don't dereference <code class="code">end()</code>.
+ </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="../spine.html">Home</a></td><td align="right" valign="top"> Chapter 11. 
+ Algorithms
+
+</td></tr></table></div></body></html>