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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>One Past the End</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.3" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt08ch19.html" title="Chapter 19. Predefined" /><link rel="prev" href="bk01pt08ch19.html" title="Chapter 19. Predefined" /><link rel="next" href="algorithms.html" title="Part IX. Algorithms" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">One Past the End</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt08ch19.html">Prev</a> </td><th width="60%" align="center">Chapter 19. Predefined</th><td width="20%" align="right"> <a accesskey="n" href="algorithms.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="iterators.predefined.end"></a>One Past the End</h2></div></div></div><p>This starts off sounding complicated, but is actually very easy,
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especially towards the end. Trust me.
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</p><p>Beginners usually have a little trouble understand the whole
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'past-the-end' thing, until they remember their early algebra classes
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(see, they <span class="emphasis"><em>told</em></span> you that stuff would come in handy!) and
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the concept of half-open ranges.
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</p><p>First, some history, and a reminder of some of the funkier rules in
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C and C++ for builtin arrays. The following rules have always been
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true for both languages:
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</p><div class="orderedlist"><ol type="1"><li><p>You can point anywhere in the array, <span class="emphasis"><em>or to the first element
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past the end of the array</em></span>. A pointer that points to one
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past the end of the array is guaranteed to be as unique as a
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pointer to somewhere inside the array, so that you can compare
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such pointers safely.
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</p></li><li><p>You can only dereference a pointer that points into an array.
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If your array pointer points outside the array -- even to just
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one past the end -- and you dereference it, Bad Things happen.
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</p></li><li><p>Strictly speaking, simply pointing anywhere else invokes
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undefined behavior. Most programs won't puke until such a
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pointer is actually dereferenced, but the standards leave that
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up to the platform.
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</p></li></ol></div><p>The reason this past-the-end addressing was allowed is to make it
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easy to write a loop to go over an entire array, e.g.,
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while (*d++ = *s++);.
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</p><p>So, when you think of two pointers delimiting an array, don't think
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of them as indexing 0 through n-1. Think of them as <span class="emphasis"><em>boundary
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markers</em></span>:
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</p><pre class="programlisting">
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beginning end
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| |
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| | This is bad. Always having to
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| | remember to add or subtract one.
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| | Off-by-one bugs very common here.
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V V
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array of N elements
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|---|---|--...--|---|---|
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| 0 | 1 | ... |N-2|N-1|
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|---|---|--...--|---|---|
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^ ^
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| |
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| | This is good. This is safe. This
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| | is guaranteed to work. Just don't
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| | dereference 'end'.
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beginning end
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</pre><p>See? Everything between the boundary markers is part of the array.
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Simple.
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</p><p>Now think back to your junior-high school algebra course, when you
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were learning how to draw graphs. Remember that a graph terminating
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with a solid dot meant, "Everything up through this point,"
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and a graph terminating with an open dot meant, "Everything up
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to, but not including, this point," respectively called closed
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and open ranges? Remember how closed ranges were written with
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brackets, <span class="emphasis"><em>[a,b]</em></span>, and open ranges were written with parentheses,
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<span class="emphasis"><em>(a,b)</em></span>?
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</p><p>The boundary markers for arrays describe a <span class="emphasis"><em>half-open range</em></span>,
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starting with (and including) the first element, and ending with (but
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not including) the last element: <span class="emphasis"><em>[beginning,end)</em></span>. See, I
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told you it would be simple in the end.
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</p><p>Iterators, and everything working with iterators, follows this same
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time-honored tradition. A container's <code class="code">begin()</code> method returns
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an iterator referring to the first element, and its <code class="code">end()</code>
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method returns a past-the-end iterator, which is guaranteed to be
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unique and comparable against any other iterator pointing into the
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middle of the container.
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</p><p>Container constructors, container methods, and algorithms, all take
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pairs of iterators describing a range of values on which to operate.
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All of these ranges are half-open ranges, so you pass the beginning
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iterator as the starting parameter, and the one-past-the-end iterator
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as the finishing parameter.
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</p><p>This generalizes very well. You can operate on sub-ranges quite
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easily this way; functions accepting a <span class="emphasis"><em>[first,last)</em></span> range
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don't know or care whether they are the boundaries of an entire {array,
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sequence, container, whatever}, or whether they only enclose a few
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elements from the center. This approach also makes zero-length
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sequences very simple to recognize: if the two endpoints compare
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equal, then the {array, sequence, container, whatever} is empty.
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</p><p>Just don't dereference <code class="code">end()</code>.
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</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt08ch19.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt08ch19.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="algorithms.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 19. Predefined </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part IX.
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Algorithms
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</td></tr></table></div></body></html>
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