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   <TITLE>libstdc++-v3 HOWTO:  Chapter 24</TITLE>
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<H1 CLASS="centered"><A NAME="top">Chapter 24:  Iterators</A></H1>

<P>Chapter 24 deals with the FORTRAN subroutines for automatically
   transforming lemmings into gold.
</P>


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<H1>Contents</H1>
<UL>
   <LI><A HREF="#1">They ain't pointers!</A>
   <LI><A HREF="#2">It ends <EM>where?</EM></A>
</UL>

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<H2><A NAME="1">They ain't pointers!</A></H2>
   <P><A HREF="../faq/index.html#5_1">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++-v3 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 <EM>pointers</EM> are
      <EM>iterators</EM>, 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 <EM>is</EM> there when using an interator class?
      Very little.  Most of the layering classes contain nothing but
      typedefs, and typedefs are &quot;meta-information&quot; 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>
   <P>Return <A HREF="#top">to top of page</A> or
      <A HREF="../faq/index.html">to the FAQ</A>.
   </P>

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<H2><A NAME="2">It ends <EM>where?</EM></A></H2>
   <P>Blah.
   </P>
   <P>Return <A HREF="#top">to top of page</A> or
      <A HREF="../faq/index.html">to the FAQ</A>.
   </P>


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<P CLASS="fineprint"><EM>
Comments and suggestions are welcome, and may be sent to
<A HREF="mailto:pme@sources.redhat.com">Phil Edwards</A> or
<A HREF="mailto:gdr@gcc.gnu.org">Gabriel Dos Reis</A>.
<BR> $Id: howto.html,v 1.5 2000/12/03 23:47:48 jsm28 Exp $
</EM></P>


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