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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 V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="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.
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Iterators
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</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt08ch19s02.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 19. Predefined"><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" title="Iterators vs. Pointers"><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>
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The following
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FAQ <a class="link" href="../faq.html#faq.iterator_as_pod" title="7.1.">entry</a> points out that
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iterators are not implemented as pointers. They are a generalization
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of pointers, but they are implemented in libstdc++ as separate
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classes.
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</p><p>Keeping that simple fact in mind as you design your code will
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prevent a whole lot of difficult-to-understand bugs.
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</p><p>You can think of it the other way 'round, even. Since iterators
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are a generalization, that means that <span class="emphasis"><em>pointers</em></span> are
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<span class="emphasis"><em>iterators</em></span>, and that pointers can be used whenever an
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iterator would be. All those functions in the Algorithms chapter
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of the Standard will work just as well on plain arrays and their
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pointers.
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</p><p>That doesn't mean that when you pass in a pointer, it gets wrapped
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into some special delegating iterator-to-pointer class with a layer
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of overhead. (If you think that's the case anywhere, you don't
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understand templates to begin with...) Oh, no; if you pass
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in a pointer, then the compiler will instantiate that template
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using T* as a type, and good old high-speed pointer arithmetic as
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its operations, so the resulting code will be doing exactly the same
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things as it would be doing if you had hand-coded it yourself (for
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the 273rd time).
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</p><p>How much overhead <span class="emphasis"><em>is</em></span> there when using an iterator class?
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Very little. Most of the layering classes contain nothing but
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typedefs, and typedefs are "meta-information" that simply
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tell the compiler some nicknames; they don't create code. That
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information gets passed down through inheritance, so while the
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compiler has to do work looking up all the names, your runtime code
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does not. (This has been a prime concern from the beginning.)
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</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.
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Iterators
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</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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