diff options
Diffstat (limited to 'libstdc++-v3/doc/html/manual/iterators.html')
| -rw-r--r-- | libstdc++-v3/doc/html/manual/iterators.html | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/libstdc++-v3/doc/html/manual/iterators.html b/libstdc++-v3/doc/html/manual/iterators.html new file mode 100644 index 000000000..8a44178b7 --- /dev/null +++ b/libstdc++-v3/doc/html/manual/iterators.html @@ -0,0 +1,130 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> +<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=" ISO C++ , library "/><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> |