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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>Chapter 10. Iterators</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="std_contents.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 width="20%" align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><th width="60%" align="center">Part II. |
| Standard Contents |
| </th><td width="20%" align="right"> <a accesskey="n" href="algorithms.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="std.iterators"></a>Chapter 10. |
| Iterators |
| <a id="id-1.3.4.8.1.1.1" class="indexterm"></a> |
| </h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><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"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.iterators.predefined"></a>Predefined</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="iterators.predefined.vs_pointers"></a>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 section 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"><div class="titlepage"><div><div><h3 class="title"><a id="iterators.predefined.end"></a>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" 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 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 width="40%" align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="std_contents.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">Interacting with C </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 11. |
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