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+ 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="containers_and_c.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>
+ You're writing some code and can't decide whether to use builtin
+ arrays or some kind of container. There are compelling reasons
+ to use one of the container classes, but you're afraid that
+ you'll eventually run into difficulties, change everything back
+ to arrays, and then have to change all the code that uses those
+ data types to keep up with the change.
+ </p><p>
+ If your code makes use of the standard algorithms, this isn't as
+ scary as it sounds. The algorithms don't know, nor care, about
+ the kind of “<span class="quote">container</span>” on which they work, since
+ the algorithms are only given endpoints to work with. For the
+ container classes, these are iterators (usually
+ <code class="code">begin()</code> and <code class="code">end()</code>, but not always).
+ For builtin arrays, these are the address of the first element
+ and the <a class="ulink" href="../24_iterators/howto.html#2" target="_top">past-the-end</a> element.
+ </p><p>
+ Some very simple wrapper functions can hide all of that from the
+ rest of the code. For example, a pair of functions called
+ <code class="code">beginof</code> can be written, one that takes an array,
+ another that takes a vector. The first returns a pointer to the
+ first element, and the second returns the vector's
+ <code class="code">begin()</code> iterator.
+ </p><p>
+ The functions should be made template functions, and should also
+ be declared inline. As pointed out in the comments in the code
+ below, this can lead to <code class="code">beginof</code> being optimized out
+ of existence, so you pay absolutely nothing in terms of increased
+ code size or execution time.
+ </p><p>
+ The result is that if all your algorithm calls look like
+ </p><pre class="programlisting">
+ std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
+ </pre><p>
+ then the type of foo can change from an array of ints to a vector
+ of ints to a deque of ints and back again, without ever changing
+ any client code.
+ </p><p>
+ This author has a collection of such functions, called
+ “<span class="quote">*of</span>” because they all extend the builtin
+ “<span class="quote">sizeof</span>”. It started with some Usenet discussions
+ on a transparent way to find the length of an array. A
+ simplified and much-reduced version for easier reading is <a class="ulink" href="wrappers_h.txt" target="_top">given here</a>.
+ </p><p>
+ Astute readers will notice two things at once: first, that the
+ container class is still a <code class="code">vector<T></code> instead
+ of a more general <code class="code">Container<T></code>. This would
+ mean that three functions for <code class="code">deque</code> would have to be
+ added, another three for <code class="code">list</code>, and so on. This is
+ due to problems with getting template resolution correct; I find
+ it easier just to give the extra three lines and avoid confusion.
+ </p><p>
+ Second, the line
+ </p><pre class="programlisting">
+ inline unsigned int lengthof (T (&)[sz]) { return sz; }
+ </pre><p>
+ looks just weird! Hint: unused parameters can be left nameless.
+ </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bitset.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>
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