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45 lines
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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>Pairs</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="utilities.html" title="Chapter 6. Utilities"/><link rel="prev" href="utilities.html" title="Chapter 6. Utilities"/><link rel="next" href="memory.html" title="Memory"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Pairs</th></tr><tr><td align="left"><a accesskey="p" href="utilities.html">Prev</a> </td><th width="60%" align="center">Chapter 6.
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Utilities
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</th><td align="right"> <a accesskey="n" href="memory.html">Next</a></td></tr></table><hr/></div><div class="section" title="Pairs"><div class="titlepage"><div><div><h2 class="title"><a id="std.util.pairs"/>Pairs</h2></div></div></div><p>The <code class="code">pair<T1,T2></code> is a simple and handy way to
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carry around a pair of objects. One is of type T1, and another of
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type T2; they may be the same type, but you don't get anything
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extra if they are. The two members can be accessed directly, as
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<code class="code">.first</code> and <code class="code">.second</code>.
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</p><p>Construction is simple. The default ctor initializes each member
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with its respective default ctor. The other simple ctor,
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</p><pre class="programlisting">
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pair (const T1& x, const T2& y);
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</pre><p>does what you think it does, <code class="code">first</code> getting <code class="code">x</code>
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and <code class="code">second</code> getting <code class="code">y</code>.
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</p><p>There is a constructor template for copying pairs of other types:
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</p><pre class="programlisting">
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template <class U, class V> pair (const pair<U,V>& p);
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</pre><p>The compiler will convert as necessary from U to T1 and from
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V to T2 in order to perform the respective initializations.
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</p><p>The comparison operators are done for you. Equality
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of two <code class="code">pair<T1,T2></code>s is defined as both <code class="code">first</code>
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members comparing equal and both <code class="code">second</code> members comparing
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equal; this simply delegates responsibility to the respective
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<code class="code">operator==</code> functions (for types like MyClass) or builtin
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comparisons (for types like int, char, etc).
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</p><p>
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The less-than operator is a bit odd the first time you see it. It
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is defined as evaluating to:
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</p><pre class="programlisting">
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x.first < y.first ||
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( !(y.first < x.first) && x.second < y.second )
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</pre><p>The other operators are not defined using the <code class="code">rel_ops</code>
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functions above, but their semantics are the same.
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</p><p>Finally, there is a template function called <code class="function">make_pair</code>
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that takes two references-to-const objects and returns an
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instance of a pair instantiated on their respective types:
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</p><pre class="programlisting">
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pair<int,MyClass> p = make_pair(4,myobject);
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</pre></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="utilities.html">Prev</a> </td><td align="center"><a accesskey="u" href="utilities.html">Up</a></td><td align="right"> <a accesskey="n" href="memory.html">Next</a></td></tr><tr><td align="left" valign="top">Chapter 6.
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Utilities
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</td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Memory</td></tr></table></div></body></html>
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