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jeremybenn |
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Interacting with C</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><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="associative.html" title="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 align="left"><a accesskey="p" href="associative.html">Prev</a> </td><th width="60%" align="center">Chapter 9.
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Containers
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</th><td align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr></table><hr/></div><div class="section" title="Interacting with C"><div class="titlepage"><div><div><h2 class="title"><a id="std.containers.c"/>Interacting with C</h2></div></div></div><div class="section" title="Containers vs. Arrays"><div class="titlepage"><div><div><h3 class="title"><a id="containers.c.vs_array"/>Containers vs. Arrays</h3></div></div></div><p>
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You're writing some code and can't decide whether to use builtin
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arrays or some kind of container. There are compelling reasons
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to use one of the container classes, but you're afraid that
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you'll eventually run into difficulties, change everything back
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to arrays, and then have to change all the code that uses those
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data types to keep up with the change.
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</p><p>
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If your code makes use of the standard algorithms, this isn't as
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scary as it sounds. The algorithms don't know, nor care, about
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the kind of <span class="quote">“<span class="quote">container</span>”</span> on which they work, since
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the algorithms are only given endpoints to work with. For the
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container classes, these are iterators (usually
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<code class="code">begin()</code> and <code class="code">end()</code>, but not always).
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For builtin arrays, these are the address of the first element
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and the <a class="link" href="iterators.html#iterators.predefined.end" title="One Past the End">past-the-end</a> element.
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</p><p>
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Some very simple wrapper functions can hide all of that from the
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rest of the code. For example, a pair of functions called
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<code class="code">beginof</code> can be written, one that takes an array,
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another that takes a vector. The first returns a pointer to the
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first element, and the second returns the vector's
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<code class="code">begin()</code> iterator.
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</p><p>
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The functions should be made template functions, and should also
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be declared inline. As pointed out in the comments in the code
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below, this can lead to <code class="code">beginof</code> being optimized out
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of existence, so you pay absolutely nothing in terms of increased
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code size or execution time.
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</p><p>
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The result is that if all your algorithm calls look like
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</p><pre class="programlisting">
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std::transform(beginof(foo), endof(foo), beginof(foo), SomeFunction);
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</pre><p>
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then the type of foo can change from an array of ints to a vector
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of ints to a deque of ints and back again, without ever changing
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any client code.
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</p><pre class="programlisting">
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// beginof
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template<typename T>
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inline typename vector<T>::iterator
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beginof(vector<T> &v)
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{ return v.begin(); }
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template<typename T, unsigned int sz>
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inline T*
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beginof(T (&array)[sz]) { return array; }
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// endof
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template<typename T>
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inline typename vector<T>::iterator
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endof(vector<T> &v)
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{ return v.end(); }
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template<typename T, unsigned int sz>
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inline T*
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endof(T (&array)[sz]) { return array + sz; }
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// lengthof
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template<typename T>
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inline typename vector<T>::size_type
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lengthof(vector<T> &v)
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{ return v.size(); }
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template<typename T, unsigned int sz>
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inline unsigned int
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lengthof(T (&)[sz]) { return sz; }
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</pre><p>
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Astute readers will notice two things at once: first, that the
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container class is still a <code class="code">vector<T></code> instead
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of a more general <code class="code">Container<T></code>. This would
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mean that three functions for <code class="code">deque</code> would have to be
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added, another three for <code class="code">list</code>, and so on. This is
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due to problems with getting template resolution correct; I find
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it easier just to give the extra three lines and avoid confusion.
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</p><p>
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Second, the line
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</p><pre class="programlisting">
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inline unsigned int lengthof (T (&)[sz]) { return sz; }
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</pre><p>
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looks just weird! Hint: unused parameters can be left nameless.
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</p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="associative.html">Prev</a> </td><td align="center"><a accesskey="u" href="containers.html">Up</a></td><td align="right"> <a accesskey="n" href="iterators.html">Next</a></td></tr><tr><td align="left" valign="top">Associative </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Chapter 10.
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Iterators
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</td></tr></table></div></body></html>
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