libstdc++-v3/doc/html/manual/iterators.html

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   3   Iterators
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   5 </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><th width="60%" align="center">Part II.
   6     Standard Contents
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   8   Iterators
   9   <a id="id-1.3.4.8.1.1.1" class="indexterm"></a>
  10 </h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="iterators.html#std.iterators.predefined">Predefined</a></span></dt><dd><dl><dt><span class="section"><a href="iterators.html#iterators.predefined.vs_pointers">Iterators vs. Pointers</a></span></dt><dt><span class="section"><a href="iterators.html#iterators.predefined.end">One Past the End</a></span></dt></dl></dd></dl></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.iterators.predefined"></a>Predefined</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="iterators.predefined.vs_pointers"></a>Iterators vs. Pointers</h3></div></div></div><p>
  11      The following
  12 FAQ <a class="link" href="../faq.html#faq.iterator_as_pod" title="7.1.">entry</a> points out that
  13 iterators are not implemented as pointers.  They are a generalization
  14 of pointers, but they are implemented in libstdc++ as separate
  15 classes.
  16    </p><p>
  17      Keeping that simple fact in mind as you design your code will
  18       prevent a whole lot of difficult-to-understand bugs.
  19    </p><p>
  20      You can think of it the other way 'round, even.  Since iterators
  21      are a generalization, that means
  22      that <span class="emphasis"><em>pointers</em></span> are
  23       <span class="emphasis"><em>iterators</em></span>, and that pointers can be used
  24      whenever an iterator would be.  All those functions in the
  25      Algorithms section of the Standard will work just as well on plain
  26      arrays and their pointers.
  27    </p><p>
  28      That doesn't mean that when you pass in a pointer, it gets
  29       wrapped into some special delegating iterator-to-pointer class
  30       with a layer of overhead.  (If you think that's the case
  31       anywhere, you don't understand templates to begin with...)  Oh,
  32       no; if you pass in a pointer, then the compiler will instantiate
  33       that template using T* as a type, and good old high-speed
  34       pointer arithmetic as its operations, so the resulting code will
  35       be doing exactly the same things as it would be doing if you had
  36       hand-coded it yourself (for the 273rd time).
  37    </p><p>
  38      How much overhead <span class="emphasis"><em>is</em></span> there when using an
  39       iterator class?  Very little.  Most of the layering classes
  40       contain nothing but typedefs, and typedefs are
  41       "meta-information" that simply tell the compiler some
  42       nicknames; they don't create code.  That information gets passed
  43       down through inheritance, so while the compiler has to do work
  44       looking up all the names, your runtime code does not.  (This has
  45       been a prime concern from the beginning.)
  46    </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="iterators.predefined.end"></a>One Past the End</h3></div></div></div><p>This starts off sounding complicated, but is actually very easy,
  47       especially towards the end.  Trust me.
  48    </p><p>Beginners usually have a little trouble understand the whole
  49       'past-the-end' thing, until they remember their early algebra classes
  50       (see, they <span class="emphasis"><em>told</em></span> you that stuff would come in handy!) and
  51       the concept of half-open ranges.
  52    </p><p>First, some history, and a reminder of some of the funkier rules in
  53       C and C++ for builtin arrays.  The following rules have always been
  54       true for both languages:
  55    </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>You can point anywhere in the array, <span class="emphasis"><em>or to the first element
  56           past the end of the array</em></span>.  A pointer that points to one
  57           past the end of the array is guaranteed to be as unique as a
  58           pointer to somewhere inside the array, so that you can compare
  59           such pointers safely.
  60         </p></li><li class="listitem"><p>You can only dereference a pointer that points into an array.
  61           If your array pointer points outside the array -- even to just
  62           one past the end -- and you dereference it, Bad Things happen.
  63         </p></li><li class="listitem"><p>Strictly speaking, simply pointing anywhere else invokes
  64           undefined behavior.  Most programs won't puke until such a
  65           pointer is actually dereferenced, but the standards leave that
  66           up to the platform.
  67         </p></li></ol></div><p>The reason this past-the-end addressing was allowed is to make it
  68       easy to write a loop to go over an entire array, e.g.,
  69       while (*d++ = *s++);.
  70    </p><p>So, when you think of two pointers delimiting an array, don't think
  71       of them as indexing 0 through n-1.  Think of them as <span class="emphasis"><em>boundary
  72       markers</em></span>:
  73    </p><pre class="programlisting">
  74
  75    beginning            end
  76      |                   |
  77      |                   |               This is bad.  Always having to
  78      |                   |               remember to add or subtract one.
  79      |                   |               Off-by-one bugs very common here.
  80      V                   V
  81         array of N elements
  82      |---|---|--...--|---|---|
  83      | 0 | 1 |  ...  |N-2|N-1|
  84      |---|---|--...--|---|---|
  85
  86      ^                       ^
  87      |                       |
  88      |                       |           This is good.  This is safe.  This
  89      |                       |           is guaranteed to work.  Just don't
  90      |                       |           dereference 'end'.
  91    beginning                end
  92
  93    </pre><p>See?  Everything between the boundary markers is chapter of the array.
  94       Simple.
  95    </p><p>Now think back to your junior-high school algebra course, when you
  96       were learning how to draw graphs.  Remember that a graph terminating
  97       with a solid dot meant, "Everything up through this point,"
  98       and a graph terminating with an open dot meant, "Everything up
  99       to, but not including, this point," respectively called closed
 100       and open ranges?  Remember how closed ranges were written with
 101       brackets, <span class="emphasis"><em>[a,b]</em></span>, and open ranges were written with parentheses,
 102       <span class="emphasis"><em>(a,b)</em></span>?
 103    </p><p>The boundary markers for arrays describe a <span class="emphasis"><em>half-open range</em></span>,
 104       starting with (and including) the first element, and ending with (but
 105       not including) the last element:  <span class="emphasis"><em>[beginning,end)</em></span>.  See, I
 106       told you it would be simple in the end.
 107    </p><p>Iterators, and everything working with iterators, follows this same
 108       time-honored tradition.  A container's <code class="code">begin()</code> method returns
 109       an iterator referring to the first element, and its <code class="code">end()</code>
 110       method returns a past-the-end iterator, which is guaranteed to be
 111       unique and comparable against any other iterator pointing into the
 112       middle of the container.
 113    </p><p>Container constructors, container methods, and algorithms, all take
 114       pairs of iterators describing a range of values on which to operate.
 115       All of these ranges are half-open ranges, so you pass the beginning
 116       iterator as the starting parameter, and the one-past-the-end iterator
 117       as the finishing parameter.
 118    </p><p>This generalizes very well.  You can operate on sub-ranges quite
 119       easily this way; functions accepting a <span class="emphasis"><em>[first,last)</em></span> range
 120       don't know or care whether they are the boundaries of an entire {array,
 121       sequence, container, whatever}, or whether they only enclose a few
 122       elements from the center.  This approach also makes zero-length
 123       sequences very simple to recognize:  if the two endpoints compare
 124       equal, then the {array, sequence, container, whatever} is empty.
 125    </p><p>Just don't dereference <code class="code">end()</code>.
 126    </p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="containers_and_c.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="std_contents.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="algorithms.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Interacting with C </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 11.
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