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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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+<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>
+</p>
<!-- ####################################################### -->
-<HR>
-<H1>Contents</H1>
-<UL>
- <LI><A HREF="#1">They ain't pointers!</A>
- <LI><A HREF="#2">It ends <EM>where?</EM></A>
-</UL>
+<hr />
+<h1>Contents</h1>
+<ul>
+ <li><a href="#1">They ain't pointers!</a></li>
+ <li><a href="#2">It ends <em>where?</em></a></li>
+</ul>
-<HR>
+<hr />
<!-- ####################################################### -->
-<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
+<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
+ 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 <EM>pointers</EM> are
- <EM>iterators</EM>, and that pointers can be used whenever an
+ </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
+ </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
+ 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?
+ </p>
+ <p>How much overhead <em>is</em> 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>
- <P>Return <A HREF="#top">to top of page</A> or
- <A HREF="../faq/index.html">to the FAQ</A>.
- </P>
+ </p>
+ <p>Return <a href="#top">to top of page</a> or
+ <a href="../faq/index.html">to the FAQ</a>.
+ </p>
+
+<hr />
+<h2><a name="2">It ends <em>where?</em></a></h2>
+ <p>This starts off sounding complicated, but is actually very easy,
+ especially towards the end. Trust me.
+ </p>
+ <p>Beginners usually have a little trouble understand the whole
+ 'past-the-end' thing, until they remember their early algebra classes
+ (see, they <em>told</em> you that stuff would come in handy!) and
+ the concept of half-open ranges.
+ </p>
+ <p>First, some history, and a reminder of some of the funkier rules in
+ C and C++ for builtin arrays. The following rules have always been
+ true for both languages:
+ </p>
+ <ol>
+ <li>You can point anywhere in the array, <em>or to the first element
+ past the end of the array</em>. A pointer that points to one
+ past the end of the array is guaranteed to be as unique as a
+ pointer to somewhere inside the array, so that you can compare
+ such pointers safely.
+ </li>
+ <li>You can only dereference a pointer that points into an array.
+ If your array pointer points outside the array -- even to just
+ one past the end -- and you dereference it, Bad Things happen.
+ </li>
+ <li>Strictly speaking, simply pointing anywhere else invokes
+ undefined behavior. Most programs won't puke until such a
+ pointer is actually dereferenced, but the standards leave that
+ up to the platform.
+ </li>
+ </ol>
+ <p>The reason this past-the-end addressing was allowed is to make it
+ easy to write a loop to go over an entire array, e.g.,
+ while (*d++ = *s++);.
+ </p>
+ <p>So, when you think of two pointers delimiting an array, don't think
+ of them as indexing 0 through n-1. Think of them as <em>boundary
+ markers</em>:
+ </p>
+ <pre>
+
+ beginning end
+ | |
+ | | This is bad. Always having to
+ | | remember to add or subtract one.
+ | | Off-by-one bugs very common here.
+ V V
+ array of N elements
+ |---|---|--...--|---|---|
+ | 0 | 1 | ... |N-2|N-1|
+ |---|---|--...--|---|---|
+
+ ^ ^
+ | |
+ | | This is good. This is safe. This
+ | | is guaranteed to work. Just don't
+ | | dereference 'end'.
+ beginning end
-<HR>
-<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>
+ </pre>
+ <p>See? Everything between the boundary markers is part of the array.
+ Simple.
+ </p>
+ <p>Now think back to your junior-high school algebra course, when you
+ were learning how to draw graphs. Remember that a graph terminating
+ with a solid dot meant, "Everything up through this point,"
+ and a graph terminating with an open dot meant, "Everything up
+ to, but not including, this point," respectively called closed
+ and open ranges? Remember how closed ranges were written with
+ brackets, <em>[a,b]</em>, and open ranges were written with parentheses,
+ <em>(a,b)</em>?
+ </p>
+ <p>The boundary markers for arrays describe a <em>half-open range</em>,
+ starting with (and including) the first element, and ending with (but
+ not including) the last element: <em>[beginning,end)</em>. See, I
+ told you it would be simple in the end.
+ </p>
+ <p>Iterators, and everything working with iterators, follows this same
+ time-honored tradition. A container's <code>begin()</code> method returns
+ an iterator referring to the first element, and its <code>end()</code>
+ method returns a past-the-end iterator, which is guaranteed to be
+ unique and comparable against any other iterator pointing into the
+ middle of the container.
+ </p>
+ <p>Container constructors, container methods, and algorithms, all take
+ pairs of iterators describing a range of values on which to operate.
+ All of these ranges are half-open ranges, so you pass the beginning
+ iterator as the starting parameter, and the one-past-the-end iterator
+ as the finishing parameter.
+ </p>
+ <p>This generalizes very well. You can operate on sub-ranges quite
+ easily this way; functions accepting a <em>[first,last)</em> range
+ don't know or care whether they are the boundaries of an entire {array,
+ sequence, container, whatever}, or whether they only enclose a few
+ elements from the center. This approach also makes zero-length
+ sequences very simple to recognize: if the two endpoints compare
+ equal, then the {array, sequence, container, whatever} is empty.
+ </p>
+ <p>Just don't dereference <code>end()</code>.
+ </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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+See <a href="../17_intro/license.html">license.html</a> for copying conditions.
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>
+<a href="mailto:libstdc++@gcc.gnu.org">the libstdc++ mailing list</a>.
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