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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>Pairs</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="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 width="20%" align="left"><a accesskey="p" href="utilities.html">Prev</a> </td><th width="60%" align="center">Chapter 6. |
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| </th><td width="20%" align="right"> <a accesskey="n" href="memory.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.util.pairs"></a>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 constructor template for copying pairs of other types: |
| </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="utilities.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 6. |
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