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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <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 V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="containers.html" title="Part VII. Containers" /><link rel="prev" href="bitset.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="bitset.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" title="Chapter 18. Interacting with C"><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" title="Containers vs. Arrays"><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">“<span class="quote">container</span>”</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="link" href="bk01pt08ch19s02.html" title="One Past the End">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><pre class="programlisting"> // beginof template<typename T> inline typename vector<T>::iterator beginof(vector<T> &v) { return v.begin(); } template<typename T, unsigned int sz> inline T* beginof(T (&array)[sz]) { return array; } // endof template<typename T> inline typename vector<T>::iterator endof(vector<T> &v) { return v.end(); } template<typename T, unsigned int sz> inline T* endof(T (&array)[sz]) { return array + sz; } // lengthof template<typename T> inline typename vector<T>::size_type lengthof(vector<T> &v) { return v.size(); } template<typename T, unsigned int sz> inline unsigned int lengthof(T (&)[sz]) { return sz; } </pre><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>