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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Pairs</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.75.2"/><meta name="keywords" content=" ISO C++ , library "/><link rel="home" href="../spine.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.
Utilities
</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
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 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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