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<H2>Bidirectional Iterators</H2>
<P><B>Library:</B>&nbsp;&nbsp;<A HREF="2-8.html">Iterators</A></P><UL>
<LI><A HREF="#sec1">Local Index</A></LI>
<LI><A HREF="#sec2">Summary</A></LI>
<LI><A HREF="#sec3">Description</A></LI>
<LI><A HREF="#sec4">Key to Iterator Requirements</A></LI>
<LI><A HREF="#sec5">Requirements for Bidirectional Iterators</A></LI>
<LI><A HREF="#sec6">See Also</A></LI>
<LI><A HREF="#sec7">Standards Conformance</A></LI>
</UL>
<A NAME="sec1"><H3>Local Index</H3></A>
No Entries
<A NAME="sec2"><H3>Summary</H3></A>
<P>An iterator that can both read and write and can traverse a container in both directions</P>
<A NAME="sec3"><H3>Description</H3></A>
<BLOCKQUOTE><HR><B>
NOTE -- For a complete discussion of iterators, see the <A HREF="iterators.html">Iterators</A> section of this reference.
</B><HR></BLOCKQUOTE>
<P>Iterators are a generalization of pointers that allow a C++ program to uniformly interact with different data structures. Bidirectional iterators can move both forwards and backwards through a container, and have the ability to both read and write data. These iterators satisfy the requirements listed below.</P>
<A NAME="sec4"><H3>Key to Iterator Requirements</H3></A>
<P>The following key pertains to the iterator descriptions listed below: </P>
<P><TABLE CELLPADDING=3 BORDER=0>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>a and b  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>values of type <SAMP>X</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>n  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>value representing a <SAMP>distance</SAMP> between two iterators</P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>u, Distance, tmp and m</SAMP>  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST">identifiers</P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>r  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>value of type <SAMP>X&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>t  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>value of type <SAMP>T</SAMP></P></TD></TR>
</TABLE></P>
<A NAME="sec5"><H3>Requirements for Bidirectional Iterators</H3></A>
<P>A bidirectional iterator must meet all the requirements listed below. Note that most of these requirements are also the requirements for forward iterators.</P>
<P><TABLE CELLPADDING=3 BORDER=0>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>X u  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST">u</SAMP> might have a singular value</P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>X()  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST">X()</SAMP> might be singular</P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>X(a)  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>copy constructor,<SAMP> a == X(a)</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>X u(a)  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>copy constructor, <SAMP>u == a</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>X u = a  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>assignment, <SAMP>u == a</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>r = a  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>assignment, <SAMP>r == a</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>a == b, a != b  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>return value convertible to <SAMP>bool</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>a-&gt;m  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>equivalent to <SAMP>(*a).m</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>*a  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>return value convertible to <SAMP>T&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>++r  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>returns <SAMP>X&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>r++  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>return value convertible to const <SAMP>X&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>*r++  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>returns <SAMP>T&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>--r  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>returns <SAMP>X&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>r--  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>return value convertible to const <SAMP>X&amp;</SAMP></P></TD></TR>
<TR CLASS="LIST"><TD VALIGN="top" CLASS="LIST"><P CLASS="LIST"><SAMP>*r--  </P></TD>
<TD CLASS="LIST"><P CLASS="LIST"></SAMP>returns <SAMP>T&amp;</SAMP></P></TD></TR>
</TABLE></P>
<P>Like forward iterators, bidirectional iterators have the condition that <SAMP>a == b</SAMP> implies <SAMP>*a== *b</SAMP>.</P>
<P>There are no restrictions on the number of passes an algorithm may make through the structure.</P>
<A NAME="sec6"><H3>See Also</H3></A>
<P><A HREF="containers.html">Containers</A>, <A HREF="iterators.html">Iterators</A>, <A HREF="forwarditerators.html">Forward Iterators</A></P>
<A NAME="sec7"><H3>Standards Conformance</H3></A>
<P><I>ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 24.1.4</I></P>

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