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6 </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>
8 FAQ <a class="link" href="../faq.html#faq.iterator_as_pod" title="7.1.">entry</a> points out that
9 iterators are not implemented as pointers. They are a generalization
10 of pointers, but they are implemented in libstdc++ as separate
12 </p><p>Keeping that simple fact in mind as you design your code will
13 prevent a whole lot of difficult-to-understand bugs.
14 </p><p>You can think of it the other way 'round, even. Since iterators
15 are a generalization, that means that <span class="emphasis"><em>pointers</em></span> are
16 <span class="emphasis"><em>iterators</em></span>, and that pointers can be used whenever an
17 iterator would be. All those functions in the Algorithms chapter
18 of the Standard will work just as well on plain arrays and their
20 </p><p>That doesn't mean that when you pass in a pointer, it gets wrapped
21 into some special delegating iterator-to-pointer class with a layer
22 of overhead. (If you think that's the case anywhere, you don't
23 understand templates to begin with...) Oh, no; if you pass
24 in a pointer, then the compiler will instantiate that template
25 using T* as a type, and good old high-speed pointer arithmetic as
26 its operations, so the resulting code will be doing exactly the same
27 things as it would be doing if you had hand-coded it yourself (for
29 </p><p>How much overhead <span class="emphasis"><em>is</em></span> there when using an iterator class?
30 Very little. Most of the layering classes contain nothing but
31 typedefs, and typedefs are "meta-information" that simply
32 tell the compiler some nicknames; they don't create code. That
33 information gets passed down through inheritance, so while the
34 compiler has to do work looking up all the names, your runtime code
35 does not. (This has been a prime concern from the beginning.)
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