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   3   Containers
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   5 </th><td width="20%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.containers.c"></a>Interacting with C</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="containers.c.vs_array"></a>Containers vs. Arrays</h3></div></div></div><p>
   6      You're writing some code and can't decide whether to use builtin
   7      arrays or some kind of container.  There are compelling reasons
   8      to use one of the container classes, but you're afraid that
   9      you'll eventually run into difficulties, change everything back
  10      to arrays, and then have to change all the code that uses those
  11      data types to keep up with the change.
  12    </p><p>
  13      If your code makes use of the standard algorithms, this isn't as
  14      scary as it sounds.  The algorithms don't know, nor care, about
  15      the kind of <span class="quote">“<span class="quote">container</span>”</span> on which they work, since
  16      the algorithms are only given endpoints to work with.  For the
  17      container classes, these are iterators (usually
  18      <code class="code">begin()</code> and <code class="code">end()</code>, but not always).
  19      For builtin arrays, these are the address of the first element
  20      and the <a class="link" href="iterators.html#iterators.predefined.end" title="One Past the End">past-the-end</a> element.
  21    </p><p>
  22      Some very simple wrapper functions can hide all of that from the
  23      rest of the code.  For example, a pair of functions called
  24      <code class="code">beginof</code> can be written, one that takes an array,
  25      another that takes a vector.  The first returns a pointer to the
  26      first element, and the second returns the vector's
  27      <code class="code">begin()</code> iterator.
  28    </p><p>
  29      The functions should be made template functions, and should also
  30      be declared inline.  As pointed out in the comments in the code
  31      below, this can lead to <code class="code">beginof</code> being optimized out
  32      of existence, so you pay absolutely nothing in terms of increased
  33      code size or execution time.
  34    </p><p>
  35      The result is that if all your algorithm calls look like
  36    </p><pre class="programlisting">
  37    std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
  38    </pre><p>
  39      then the type of foo can change from an array of ints to a vector
  40      of ints to a deque of ints and back again, without ever changing
  41      any client code.
  42    </p><pre class="programlisting">
  43 // beginof
  44 template&lt;typename T&gt;
  45   inline typename vector&lt;T&gt;::iterator
  46   beginof(vector&lt;T&gt; &amp;v)
  47   { return v.begin(); }
  48
  49 template&lt;typename T, unsigned int sz&gt;
  50   inline T*
  51   beginof(T (&amp;array)[sz]) { return array; }
  52
  53 // endof
  54 template&lt;typename T&gt;
  55   inline typename vector&lt;T&gt;::iterator
  56   endof(vector&lt;T&gt; &amp;v)
  57   { return v.end(); }
  58
  59 template&lt;typename T, unsigned int sz&gt;
  60   inline T*
  61   endof(T (&amp;array)[sz]) { return array + sz; }
  62
  63 // lengthof
  64 template&lt;typename T&gt;
  65   inline typename vector&lt;T&gt;::size_type
  66   lengthof(vector&lt;T&gt; &amp;v)
  67   { return v.size(); }
  68
  69 template&lt;typename T, unsigned int sz&gt;
  70   inline unsigned int
  71   lengthof(T (&amp;)[sz]) { return sz; }
  72 </pre><p>
  73      Astute readers will notice two things at once: first, that the
  74      container class is still a <code class="code">vector&lt;T&gt;</code> instead
  75      of a more general <code class="code">Container&lt;T&gt;</code>.  This would
  76      mean that three functions for <code class="code">deque</code> would have to be
  77      added, another three for <code class="code">list</code>, and so on.  This is
  78      due to problems with getting template resolution correct; I find
  79      it easier just to give the extra three lines and avoid confusion.
  80    </p><p>
  81      Second, the line
  82    </p><pre class="programlisting">
  83     inline unsigned int lengthof (T (&amp;)[sz]) { return sz; }
  84    </pre><p>
  85      looks just weird!  Hint:  unused parameters can be left nameless.
  86    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="unordered_associative.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="containers.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Unordered Associative </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 10.
  87   Iterators
  88
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