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