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   3 <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 18. Interacting with C</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content="&#10;      ISO C++&#10;    , &#10;      library&#10;    " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="containers.html" title="Part VII. Containers" /><link rel="prev" href="bk01pt07ch17s02.html" title="bitset" /><link rel="next" href="iterators.html" title="Part VIII. Iterators" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 18. Interacting with C</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt07ch17s02.html">Prev</a> </td><th width="60%" align="center">Part VII. Containers</th><td width="20%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr></table><hr /></div><div class="chapter" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title"><a id="manual.containers.c"></a>Chapter 18. Interacting with C</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="bk01pt07ch18.html#containers.c.vs_array">Containers vs. Arrays</a></span></dt></dl></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="containers.c.vs_array"></a>Containers vs. Arrays</h2></div></div></div><p>
   4      You're writing some code and can't decide whether to use builtin
   5      arrays or some kind of container.  There are compelling reasons
   6      to use one of the container classes, but you're afraid that
   7      you'll eventually run into difficulties, change everything back
   8      to arrays, and then have to change all the code that uses those
   9      data types to keep up with the change.
  10    </p><p>
  11      If your code makes use of the standard algorithms, this isn't as
  12      scary as it sounds.  The algorithms don't know, nor care, about
  13      the kind of “<span class="quote">container</span>” on which they work, since
  14      the algorithms are only given endpoints to work with.  For the
  15      container classes, these are iterators (usually
  16      <code class="code">begin()</code> and <code class="code">end()</code>, but not always).
  17      For builtin arrays, these are the address of the first element
  18      and the <a class="ulink" href="../24_iterators/howto.html#2" target="_top">past-the-end</a> element.
  19    </p><p>
  20      Some very simple wrapper functions can hide all of that from the
  21      rest of the code.  For example, a pair of functions called
  22      <code class="code">beginof</code> can be written, one that takes an array,
  23      another that takes a vector.  The first returns a pointer to the
  24      first element, and the second returns the vector's
  25      <code class="code">begin()</code> iterator.
  26    </p><p>
  27      The functions should be made template functions, and should also
  28      be declared inline.  As pointed out in the comments in the code
  29      below, this can lead to <code class="code">beginof</code> being optimized out
  30      of existence, so you pay absolutely nothing in terms of increased
  31      code size or execution time.
  32    </p><p>
  33      The result is that if all your algorithm calls look like
  34    </p><pre class="programlisting">
  35    std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
  36    </pre><p>
  37      then the type of foo can change from an array of ints to a vector
  38      of ints to a deque of ints and back again, without ever changing
  39      any client code.
  40    </p><p>
  41      This author has a collection of such functions, called
  42      “<span class="quote">*of</span>” because they all extend the builtin
  43      “<span class="quote">sizeof</span>”.  It started with some Usenet discussions
  44      on a transparent way to find the length of an array.  A
  45      simplified and much-reduced version for easier reading is <a class="ulink" href="wrappers_h.txt" target="_top">given here</a>.
  46    </p><p>
  47      Astute readers will notice two things at once: first, that the
  48      container class is still a <code class="code">vector&lt;T&gt;</code> instead
  49      of a more general <code class="code">Container&lt;T&gt;</code>.  This would
  50      mean that three functions for <code class="code">deque</code> would have to be
  51      added, another three for <code class="code">list</code>, and so on.  This is
  52      due to problems with getting template resolution correct; I find
  53      it easier just to give the extra three lines and avoid confusion.
  54    </p><p>
  55      Second, the line
  56    </p><pre class="programlisting">
  57     inline unsigned int lengthof (T (&amp;)[sz]) { return sz; }
  58    </pre><p>
  59      looks just weird!  Hint:  unused parameters can be left nameless.
  60    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt07ch17s02.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">bitset </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part VIII. Iterators</td></tr></table></div></body></html>