--- /dev/null
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!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>
projects / msp430-gcc.git / blobdiff