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96 lines
3.6 KiB
HTML
96 lines
3.6 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<META NAME="AUTHOR" CONTENT="pme@sourceware.cygnus.com (Phil Edwards)">
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<META NAME="KEYWORDS" CONTENT="HOWTO, libstdc++, egcs, g++, libg++, STL">
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<META NAME="DESCRIPTION" CONTENT="HOWTO for the libstdc++ chapter 24.">
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<META NAME="GENERATOR" CONTENT="vi and eight fingers">
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<TITLE>libstdc++-v3 HOWTO: Chapter 24</TITLE>
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<LINK REL="home" HREF="http://sourceware.cygnus.com/libstdc++/docs/24_iterators/">
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<LINK REL=StyleSheet HREF="../lib3styles.css">
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<!-- $Id: howto.html,v 1.4 1999/12/15 16:57:06 pme Exp $ -->
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</HEAD>
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<BODY>
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<H1 CLASS="centered"><A NAME="top">Chapter 24: Iterators</A></H1>
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<P>Chapter 24 deals with the FORTRAN subroutines for automatically
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transforming lemmings into gold.
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</P>
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<!-- ####################################################### -->
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<HR>
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<H1>Contents</H1>
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<UL>
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<LI><A HREF="#1">They ain't pointers!</A>
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<LI><A HREF="#2">Topic</A>
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</UL>
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<HR>
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<!-- ####################################################### -->
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<H2><A NAME="1">They ain't pointers!</A></H2>
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<P><A HREF="../faq/index.html#5_1">FAQ 5.1</A> points out that iterators
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are not implemented as pointers. They are a generalization of
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pointers, but they are implemented in libstdc++-v3 as separate classes.
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</P>
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<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>
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<P>You can think of it the other way 'round, even. Since iterators
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are a generalization, that means that <EM>pointers</EM> are
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<EM>iterators</EM>, 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>
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<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>
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<P>How much overhead <EM>is</EM> there when using an interator 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>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<HR>
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<H2><A NAME="2">Topic</A></H2>
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<P>Blah.
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</P>
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<P>Return <A HREF="#top">to top of page</A> or
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<A HREF="../faq/index.html">to the FAQ</A>.
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</P>
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<!-- ####################################################### -->
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<HR>
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<P CLASS="fineprint"><EM>
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Comments and suggestions are welcome, and may be sent to
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<A HREF="mailto:pme@sourceware.cygnus.com">Phil Edwards</A> or
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<A HREF="mailto:gdr@egcs.cygnus.com">Gabriel Dos Reis</A>.
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<BR> $Id: howto.html,v 1.4 1999/12/15 16:57:06 pme Exp $
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</EM></P>
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</BODY>
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</HTML>
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