1 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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 V1.74.3" /><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.
6 </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>
7 You're writing some code and can't decide whether to use builtin
8 arrays or some kind of container. There are compelling reasons
9 to use one of the container classes, but you're afraid that
10 you'll eventually run into difficulties, change everything back
11 to arrays, and then have to change all the code that uses those
12 data types to keep up with the change.
14 If your code makes use of the standard algorithms, this isn't as
15 scary as it sounds. The algorithms don't know, nor care, about
16 the kind of “<span class="quote">container</span>” on which they work, since
17 the algorithms are only given endpoints to work with. For the
18 container classes, these are iterators (usually
19 <code class="code">begin()</code> and <code class="code">end()</code>, but not always).
20 For builtin arrays, these are the address of the first element
21 and the <a class="ulink" href="../24_iterators/howto.html#2" target="_top">past-the-end</a> element.
23 Some very simple wrapper functions can hide all of that from the
24 rest of the code. For example, a pair of functions called
25 <code class="code">beginof</code> can be written, one that takes an array,
26 another that takes a vector. The first returns a pointer to the
27 first element, and the second returns the vector's
28 <code class="code">begin()</code> iterator.
30 The functions should be made template functions, and should also
31 be declared inline. As pointed out in the comments in the code
32 below, this can lead to <code class="code">beginof</code> being optimized out
33 of existence, so you pay absolutely nothing in terms of increased
34 code size or execution time.
36 The result is that if all your algorithm calls look like
37 </p><pre class="programlisting">
38 std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
40 then the type of foo can change from an array of ints to a vector
41 of ints to a deque of ints and back again, without ever changing
44 This author has a collection of such functions, called
45 “<span class="quote">*of</span>” because they all extend the builtin
46 “<span class="quote">sizeof</span>”. It started with some Usenet discussions
47 on a transparent way to find the length of an array. A
48 simplified and much-reduced version for easier reading is <a class="ulink" href="wrappers_h.txt" target="_top">given here</a>.
50 Astute readers will notice two things at once: first, that the
51 container class is still a <code class="code">vector<T></code> instead
52 of a more general <code class="code">Container<T></code>. This would
53 mean that three functions for <code class="code">deque</code> would have to be
54 added, another three for <code class="code">list</code>, and so on. This is
55 due to problems with getting template resolution correct; I find
56 it easier just to give the extra three lines and avoid confusion.
59 </p><pre class="programlisting">
60 inline unsigned int lengthof (T (&)[sz]) { return sz; }
62 looks just weird! Hint: unused parameters can be left nameless.
63 </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.
66 </td></tr></table></div></body></html>