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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>Chapter 19. Predefined</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="iterators.html" title="Part VIII. Iterators" /><link rel="prev" href="iterators.html" title="Part VIII. Iterators" /><link rel="next" href="bk01pt08ch19s02.html" title="One Past the End" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 19. Predefined</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="iterators.html">Prev</a> </td><th width="60%" align="center">Part VIII.
+ Iterators
+
+</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt08ch19s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.iterators.predefined"></a>Chapter 19. Predefined</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="bk01pt08ch19.html#iterators.predefined.vs_pointers">Iterators vs. Pointers</a></span></dt><dt><span class="sect1"><a href="bk01pt08ch19s02.html">One Past the End</a></span></dt></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="iterators.predefined.vs_pointers"></a>Iterators vs. Pointers</h2></div></div></div><p><a class="ulink" href="../faq/index.html#5_1" target="_top">FAQ 5.1</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 chapter
+ 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><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="iterators.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="iterators.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt08ch19s02.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part VIII.
+ Iterators
+
+ </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> One Past the End</td></tr></table></div></body></html>
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