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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 10. Pairs</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="utilities.html" title="Part IV. Utilities" /><link rel="prev" href="functors.html" title="Chapter 9. Functors" /><link rel="next" href="memory.html" title="Chapter 11. Memory" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 10. Pairs</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="functors.html">Prev</a> </td><th width="60%" align="center">Part IV. Utilities </th><td width="20%" align="right"> <a accesskey="n" href="memory.html">Next</a></td></tr></table><hr /></div><div class="chapter" title="Chapter 10. Pairs"><div class="titlepage"><div><div><h2 class="title"><a id="manual.util.pairs"></a>Chapter 10. Pairs</h2></div></div></div><p>The <code class="code">pair<T1,T2></code> is a simple and handy way to carry around a pair of objects. One is of type T1, and another of type T2; they may be the same type, but you don't get anything extra if they are. The two members can be accessed directly, as <code class="code">.first</code> and <code class="code">.second</code>. </p><p>Construction is simple. The default ctor initializes each member with its respective default ctor. The other simple ctor, </p><pre class="programlisting"> pair (const T1& x, const T2& y); </pre><p>does what you think it does, <code class="code">first</code> getting <code class="code">x</code> and <code class="code">second</code> getting <code class="code">y</code>. </p><p>There is a copy constructor, but it requires that your compiler handle member function templates: </p><pre class="programlisting"> template <class U, class V> pair (const pair<U,V>& p); </pre><p>The compiler will convert as necessary from U to T1 and from V to T2 in order to perform the respective initializations. </p><p>The comparison operators are done for you. Equality of two <code class="code">pair<T1,T2></code>s is defined as both <code class="code">first</code> members comparing equal and both <code class="code">second</code> members comparing equal; this simply delegates responsibility to the respective <code class="code">operator==</code> functions (for types like MyClass) or builtin comparisons (for types like int, char, etc). </p><p> The less-than operator is a bit odd the first time you see it. It is defined as evaluating to: </p><pre class="programlisting"> x.first < y.first || ( !(y.first < x.first) && x.second < y.second ) </pre><p>The other operators are not defined using the <code class="code">rel_ops</code> functions above, but their semantics are the same. </p><p>Finally, there is a template function called <code class="function">make_pair</code> that takes two references-to-const objects and returns an instance of a pair instantiated on their respective types: </p><pre class="programlisting"> pair<int,MyClass> p = make_pair(4,myobject); </pre></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="functors.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="utilities.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="memory.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 9. Functors </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 11. Memory</td></tr></table></div></body></html>
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