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| <!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>Interacting with C</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><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="unordered_associative.html" title="Unordered Associative" /><link rel="next" href="iterators.html" title="Chapter 10. Iterators" /></head><body><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="unordered_associative.html">Prev</a> </td><th width="60%" align="center">Chapter 9. |
| Containers |
| |
| </th><td width="20%" align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.containers.c"></a>Interacting with C</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="containers.c.vs_array"></a>Containers vs. Arrays</h3></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="iterators.html#iterators.predefined.end" 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="unordered_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">Unordered 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. |
| Iterators |
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