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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 V1.74.0" /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><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,
       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 type="1"><li><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><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><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 part 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 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.
   Algorithms

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	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
	<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 V1.74.0" /><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,
	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 type="1"><li><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><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><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 part 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 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.
	Algorithms

	</td></tr></table></div></body></html>