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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 V1.75.2" /><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.�� 4 Containers 5 6</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> 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. 13 </p><p> 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">���<span class="quote">container</span>���</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="link" href="bk01pt08ch19s02.html" title="One Past the End">past-the-end</a> element. 22 </p><p> 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. 29 </p><p> 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. 35 </p><p> 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); 39 </pre><p> 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 42 any client code. 43 </p><pre class="programlisting"> 44// beginof 45template<typename T> 46 inline typename vector<T>::iterator 47 beginof(vector<T> &v) 48 { return v.begin(); } 49 50template<typename T, unsigned int sz> 51 inline T* 52 beginof(T (&array)[sz]) { return array; } 53 54// endof 55template<typename T> 56 inline typename vector<T>::iterator 57 endof(vector<T> &v) 58 { return v.end(); } 59 60template<typename T, unsigned int sz> 61 inline T* 62 endof(T (&array)[sz]) { return array + sz; } 63 64// lengthof 65template<typename T> 66 inline typename vector<T>::size_type 67 lengthof(vector<T> &v) 68 { return v.size(); } 69 70template<typename T, unsigned int sz> 71 inline unsigned int 72 lengthof(T (&)[sz]) { return sz; } 73</pre><p> 74 Astute readers will notice two things at once: first, that the 75 container class is still a <code class="code">vector<T></code> instead 76 of a more general <code class="code">Container<T></code>. This would 77 mean that three functions for <code class="code">deque</code> would have to be 78 added, another three for <code class="code">list</code>, and so on. This is 79 due to problems with getting template resolution correct; I find 80 it easier just to give the extra three lines and avoid confusion. 81 </p><p> 82 Second, the line 83 </p><pre class="programlisting"> 84 inline unsigned int lengthof (T (&)[sz]) { return sz; } 85 </pre><p> 86 looks just weird! Hint: unused parameters can be left nameless. 87 </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.�� 88 Iterators 89 90</td></tr></table></div></body></html> 91