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  14 </th><td width="20%" 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" style="clear: both"><a name="std.util.pairs"></a>Pairs</h2></div></div></div><p>The <code class="code">pair&lt;T1,T2&gt;</code> is a simple and handy way to
  15       carry around a pair of objects.  One is of type T1, and another of
  16       type T2; they may be the same type, but you don't get anything
  17       extra if they are.  The two members can be accessed directly, as
  18       <code class="code">.first</code> and <code class="code">.second</code>.
  19    </p><p>Construction is simple.  The default ctor initializes each member
  20       with its respective default ctor.  The other simple ctor,
  21    </p><pre class="programlisting">
  22     pair (const T1&amp; x, const T2&amp; y);
  23    </pre><p>does what you think it does, <code class="code">first</code> getting <code class="code">x</code>
  24       and <code class="code">second</code> getting <code class="code">y</code>.
  25    </p><p>There is a constructor template for copying pairs of other types:
  26    </p><pre class="programlisting">
  27     template &lt;class U, class V&gt; pair (const pair&lt;U,V&gt;&amp; p);
  28    </pre><p>The compiler will convert as necessary from U to T1 and from
  29       V to T2 in order to perform the respective initializations.
  30    </p><p>The comparison operators are done for you.  Equality
  31       of two <code class="code">pair&lt;T1,T2&gt;</code>s is defined as both <code class="code">first</code>
  32       members comparing equal and both <code class="code">second</code> members comparing
  33       equal; this simply delegates responsibility to the respective
  34       <code class="code">operator==</code> functions (for types like MyClass) or builtin
  35       comparisons (for types like int, char, etc).
  36    </p><p>
  37       The less-than operator is a bit odd the first time you see it.  It
  38       is defined as evaluating to:
  39    </p><pre class="programlisting">
  40     x.first  &lt;  y.first  ||
  41         ( !(y.first  &lt;  x.first)  &amp;&amp;  x.second  &lt;  y.second )
  42    </pre><p>The other operators are not defined using the <code class="code">rel_ops</code>
  43       functions above, but their semantics are the same.
  44    </p><p>Finally, there is a template function called <code class="function">make_pair</code>
  45       that takes two references-to-const objects and returns an
  46       instance of a pair instantiated on their respective types:
  47    </p><pre class="programlisting">
  48     pair&lt;int,MyClass&gt; p = make_pair(4,myobject);
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