URL
https://opencores.org/ocsvn/openrisc/openrisc/trunk
Subversion Repositories openrisc
[/] [openrisc/] [trunk/] [gnu-stable/] [gcc-4.5.1/] [libstdc++-v3/] [doc/] [html/] [manual/] [bk01pt08ch19s02.html] - Rev 816
Go to most recent revision | Compare with Previous | Blame | View Log
<?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 V1.75.2" /><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" title="One Past the End"><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 class="orderedlist" type="1"><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 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>
Go to most recent revision | Compare with Previous | Blame | View Log