libstdc++-v3/docs/html/24_iterators/howto.html

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   9    <meta name="AUTHOR" content="pme@gcc.gnu.org (Phil Edwards)" />
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  13    <title>libstdc++-v3 HOWTO:  Chapter 24</title>
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  17
  18 <h1 class="centered"><a name="top">Chapter 24:  Iterators</a></h1>
  19
  20 <p>Chapter 24 deals with the FORTRAN subroutines for automatically
  21    transforming lemmings into gold.
  22 </p>
  23
  24
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  26 <hr />
  27 <h1>Contents</h1>
  28 <ul>
  29    <li><a href="#1">They ain't pointers!</a></li>
  30    <li><a href="#2">It ends <em>where?</em></a></li>
  31 </ul>
  32
  33 <hr />
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  36
  37 <h2><a name="1">They ain't pointers!</a></h2>
  38    <p><a href="../faq/index.html#5_1">FAQ 5.1</a> points out that iterators
  39       are not implemented as pointers.  They are a generalization of
  40       pointers, but they are implemented in libstdc++-v3 as separate classes.
  41    </p>
  42    <p>Keeping that simple fact in mind as you design your code will
  43       prevent a whole lot of difficult-to-understand bugs.
  44    </p>
  45    <p>You can think of it the other way 'round, even.  Since iterators
  46       are a generalization, that means that <em>pointers</em> are
  47       <em>iterators</em>, and that pointers can be used whenever an
  48       iterator would be.  All those functions in the Algorithms chapter
  49       of the Standard will work just as well on plain arrays and their
  50       pointers.
  51    </p>
  52    <p>That doesn't mean that when you pass in a pointer, it gets wrapped
  53       into some special delegating iterator-to-pointer class with a layer
  54       of overhead.  (If you think that's the case anywhere, you don't
  55       understand templates to begin with...)  Oh, no; if you pass
  56       in a pointer, then the compiler will instantiate that template
  57       using T* as a type, and good old high-speed pointer arithmetic as
  58       its operations, so the resulting code will be doing exactly the same
  59       things as it would be doing if you had hand-coded it yourself (for
  60       the 273rd time).
  61    </p>
  62    <p>How much overhead <em>is</em> there when using an interator class?
  63       Very little.  Most of the layering classes contain nothing but
  64       typedefs, and typedefs are &quot;meta-information&quot; that simply
  65       tell the compiler some nicknames; they don't create code.  That
  66       information gets passed down through inheritance, so while the
  67       compiler has to do work looking up all the names, your runtime code
  68       does not.  (This has been a prime concern from the beginning.)
  69    </p>
  70    <p>Return <a href="#top">to top of page</a> or
  71       <a href="../faq/index.html">to the FAQ</a>.
  72    </p>
  73
  74 <hr />
  75 <h2><a name="2">It ends <em>where?</em></a></h2>
  76    <p>This starts off sounding complicated, but is actually very easy,
  77       especially towards the end.  Trust me.
  78    </p>
  79    <p>Beginners usually have a little trouble understand the whole
  80       'past-the-end' thing, until they remember their early algebra classes
  81       (see, they <em>told</em> you that stuff would come in handy!) and
  82       the concept of half-open ranges.
  83    </p>
  84    <p>First, some history, and a reminder of some of the funkier rules in
  85       C and C++ for builtin arrays.  The following rules have always been
  86       true for both languages:
  87    </p>
  88    <ol>
  89       <li>You can point anywhere in the array, <em>or to the first element
  90           past the end of the array</em>.  A pointer that points to one
  91           past the end of the array is guaranteed to be as unique as a
  92           pointer to somewhere inside the array, so that you can compare
  93           such pointers safely.
  94       </li>
  95       <li>You can only dereference a pointer that points into an array.
  96           If your array pointer points outside the array -- even to just
  97           one past the end -- and you dereference it, Bad Things happen.
  98       </li>
  99       <li>Strictly speaking, simply pointing anywhere else invokes
 100           undefined behavior.  Most programs won't puke until such a
 101           pointer is actually dereferenced, but the standards leave that
 102           up to the platform.
 103       </li>
 104    </ol>
 105    <p>The reason this past-the-end addressing was allowed is to make it
 106       easy to write a loop to go over an entire array, e.g.,
 107       while (*d++ = *s++);.
 108    </p>
 109    <p>So, when you think of two pointers delimiting an array, don't think
 110       of them as indexing 0 through n-1.  Think of them as <em>boundary
 111       markers</em>:
 112    </p>
 113    <pre>
 114
 115    beginning            end
 116      |                   |
 117      |                   |               This is bad.  Always having to
 118      |                   |               remember to add or subtract one.
 119      |                   |               Off-by-one bugs very common here.
 120      V                   V
 121         array of N elements
 122      |---|---|--...--|---|---|
 123      | 0 | 1 |  ...  |N-2|N-1|
 124      |---|---|--...--|---|---|
 125
 126      ^                       ^
 127      |                       |
 128      |                       |           This is good.  This is safe.  This
 129      |                       |           is guaranteed to work.  Just don't
 130      |                       |           dereference 'end'.
 131    beginning                end
 132
 133    </pre>
 134    <p>See?  Everything between the boundary markers is part of the array.
 135       Simple.
 136    </p>
 137    <p>Now think back to your junior-high school algebra course, when you
 138       were learning how to draw graphs.  Remember that a graph terminating
 139       with a solid dot meant, &quot;Everything up through this point,&quot;
 140       and a graph terminating with an open dot meant, &quot;Everything up
 141       to, but not including, this point,&quot; respectively called closed
 142       and open ranges?  Remember how closed ranges were written with
 143       brackets, <em>[a,b]</em>, and open ranges were written with parentheses,
 144       <em>(a,b)</em>?
 145    </p>
 146    <p>The boundary markers for arrays describe a <em>half-open range</em>,
 147       starting with (and including) the first element, and ending with (but
 148       not including) the last element:  <em>[beginning,end)</em>.  See, I
 149       told you it would be simple in the end.
 150    </p>
 151    <p>Iterators, and everything working with iterators, follows this same
 152       time-honored tradition.  A container's <code>begin()</code> method returns
 153       an iterator referring to the first element, and its <code>end()</code>
 154       method returns a past-the-end iterator, which is guaranteed to be
 155       unique and comparable against any other iterator pointing into the
 156       middle of the container.
 157    </p>
 158    <p>Container constructors, container methods, and algorithms, all take
 159       pairs of iterators describing a range of values on which to operate.
 160       All of these ranges are half-open ranges, so you pass the beginning
 161       iterator as the starting parameter, and the one-past-the-end iterator
 162       as the finishing parameter.
 163    </p>
 164    <p>This generalizes very well.  You can operate on sub-ranges quite
 165       easily this way; functions accepting a <em>[first,last)</em> range
 166       don't know or care whether they are the boundaries of an entire {array,
 167       sequence, container, whatever}, or whether they only enclose a few
 168       elements from the center.  This approach also makes zero-length
 169       sequences very simple to recognize:  if the two endpoints compare
 170       equal, then the {array, sequence, container, whatever} is empty.
 171    </p>
 172    <p>Just don't dereference <code>end()</code>.
 173    </p>
 174    <p>Return <a href="#top">to top of page</a> or
 175       <a href="../faq/index.html">to the FAQ</a>.
 176    </p>
 177
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