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  11     "><link rel="home" href="../index.html" title="The GNU C++ Library"><link rel="up" href="containers.html" title="Chapter 9.  Containers"><link rel="prev" href="associative.html" title="Associative"><link rel="next" href="iterators.html" title="Chapter 10.  Iterators"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Interacting with C</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="associative.html">Prev</a> </td><th width="60%" align="center">Chapter 9.
  12   Containers
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  14 </th><td width="20%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr></table><hr></div><div class="section" title="Interacting with C"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="std.containers.c"></a>Interacting with C</h2></div></div></div><div class="section" title="Containers vs. Arrays"><div class="titlepage"><div><div><h3 class="title"><a name="containers.c.vs_array"></a>Containers vs. Arrays</h3></div></div></div><p>
  15      You're writing some code and can't decide whether to use builtin
  16      arrays or some kind of container.  There are compelling reasons
  17      to use one of the container classes, but you're afraid that
  18      you'll eventually run into difficulties, change everything back
  19      to arrays, and then have to change all the code that uses those
  20      data types to keep up with the change.
  21    </p><p>
  22      If your code makes use of the standard algorithms, this isn't as
  23      scary as it sounds.  The algorithms don't know, nor care, about
  24      the kind of <span class="quote">“<span class="quote">container</span>”</span> on which they work, since
  25      the algorithms are only given endpoints to work with.  For the
  26      container classes, these are iterators (usually
  27      <code class="code">begin()</code> and <code class="code">end()</code>, but not always).
  28      For builtin arrays, these are the address of the first element
  29      and the <a class="link" href="iterators.html#iterators.predefined.end" title="One Past the End">past-the-end</a> element.
  30    </p><p>
  31      Some very simple wrapper functions can hide all of that from the
  32      rest of the code.  For example, a pair of functions called
  33      <code class="code">beginof</code> can be written, one that takes an array,
  34      another that takes a vector.  The first returns a pointer to the
  35      first element, and the second returns the vector's
  36      <code class="code">begin()</code> iterator.
  37    </p><p>
  38      The functions should be made template functions, and should also
  39      be declared inline.  As pointed out in the comments in the code
  40      below, this can lead to <code class="code">beginof</code> being optimized out
  41      of existence, so you pay absolutely nothing in terms of increased
  42      code size or execution time.
  43    </p><p>
  44      The result is that if all your algorithm calls look like
  45    </p><pre class="programlisting">
  46    std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
  47    </pre><p>
  48      then the type of foo can change from an array of ints to a vector
  49      of ints to a deque of ints and back again, without ever changing
  50      any client code.
  51    </p><pre class="programlisting">
  52 // beginof
  53 template&lt;typename T&gt;
  54   inline typename vector&lt;T&gt;::iterator
  55   beginof(vector&lt;T&gt; &amp;v)
  56   { return v.begin(); }
  57
  58 template&lt;typename T, unsigned int sz&gt;
  59   inline T*
  60   beginof(T (&amp;array)[sz]) { return array; }
  61
  62 // endof
  63 template&lt;typename T&gt;
  64   inline typename vector&lt;T&gt;::iterator
  65   endof(vector&lt;T&gt; &amp;v)
  66   { return v.end(); }
  67
  68 template&lt;typename T, unsigned int sz&gt;
  69   inline T*
  70   endof(T (&amp;array)[sz]) { return array + sz; }
  71
  72 // lengthof
  73 template&lt;typename T&gt;
  74   inline typename vector&lt;T&gt;::size_type
  75   lengthof(vector&lt;T&gt; &amp;v)
  76   { return v.size(); }
  77
  78 template&lt;typename T, unsigned int sz&gt;
  79   inline unsigned int
  80   lengthof(T (&amp;)[sz]) { return sz; }
  81 </pre><p>
  82      Astute readers will notice two things at once: first, that the
  83      container class is still a <code class="code">vector&lt;T&gt;</code> instead
  84      of a more general <code class="code">Container&lt;T&gt;</code>.  This would
  85      mean that three functions for <code class="code">deque</code> would have to be
  86      added, another three for <code class="code">list</code>, and so on.  This is
  87      due to problems with getting template resolution correct; I find
  88      it easier just to give the extra three lines and avoid confusion.
  89    </p><p>
  90      Second, the line
  91    </p><pre class="programlisting">
  92     inline unsigned int lengthof (T (&amp;)[sz]) { return sz; }
  93    </pre><p>
  94      looks just weird!  Hint:  unused parameters can be left nameless.
  95    </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="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">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.
  96   Iterators
  97
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