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<H2>8.1 The set Data Abstraction</H2>
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<P>A <B><I><A HREF="../stdlibref/set.html">set</A></I></B> is a collection of values. Although the abstract concept of a set does not necessarily imply an ordered collection, the standard library <B><I>set</I></B> datatype is always ordered. </P>
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NOTE --  If necessary, a collection of values that cannot be ordered can be maintained in an alternative data structure, such as a <B><I><A HREF="../stdlibref/list.html">list</A></I></B>.
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<P>Because the container used to implement the <B><I><A HREF="../stdlibref/set.html">set</A></I></B> data structure maintains values in an ordered representation, <B><I>set</I></B>s are optimized for insertion and removal of elements, and for testing to see whether a particular value is contained in the collection. Each of these operations can be performed in a logarithmic number of steps, whereas for a <B><I><A HREF="../stdlibref/list.html">list</A></I></B>, <B><I><A HREF="../stdlibref/vector.html">vector</A></I></B>, or <B><I><A HREF="../stdlibref/deque.html">deque</A></I></B>, each operation requires in the worst case an examination of every element held by the container. For this reason, <B><I><A HREF="../stdlibref/set.html">std::set</A></I></B> should be the data structure of choice in any problem that emphasizes insertion, removal, and test for inclusion of values. Like a <B><I>list</I></B>, a <B><I>set</I></B> is not limited in size, but rather expands and contracts as elements are added to or removed from the collection.</P>
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<P>There are two varieties of <B><I><A HREF="../stdlibref/set.html">set</A></I></B>s provided by the C++ Standard Library. In the <B><I>set</I></B> container, every element is unique, and insertions of values that are already contained in the <B><I>set</I></B> are ignored. In the <B><I><A HREF="../stdlibref/multiset.html">multiset</A></I></B> container, on the other hand, multiple occurrences of the same value are permitted.</P>
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NOTE -- In other programming languages, a multiset is sometimes referred to as a bag.
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<A NAME="811"><H3>8.1.1 Include Files</H3></A>
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<P>Whenever you use a <B><I><A HREF="../stdlibref/set.html">set</A></I></B> or a <B><I><A HREF="../stdlibref/multiset.html">multiset</A></I></B>, you must include the set header file:</P>

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#include &lt;set&gt;
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