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<?xml version="1.0"?>
<concept name="BidirectionalIterator" category="Iterator"><!--
Based on concepts from the SGI Standard Template Library documentation:
Copyright (c) 1996-1999
Silicon Graphics Computer Systems, Inc.
Copyright (c) 1994
Hewlett-Packard Company
--><!--
Copyright 2000-2001 University of Notre Dame du Lac.
Copyright 2001-2002 Indiana University.
Some concepts based on versions from the MTL draft manual and Boost Graph
and Property Map documentation:
Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000
-->
<param name="Iter" role="iterator-type"/>
<use-header name="iterator"/>
<models-sentence>The iterator type <arg num="1"/> must be a model of <self/>.</models-sentence>
<description>
<para>A bidirectional iterator is an iterator that can read through a sequence
of values. It can move in either direction through the sequence, and can
be either mutable (data pointed to by it can be changed) or not mutable.</para>
<para>An iterator represents a position in a sequence. Therefore, the
iterator can point into the sequence (returning a value when dereferenced
and being incrementable), or be off-the-end (and not dereferenceable or
incrementable).</para>
</description>
<associated-type name="value_type">
<get-member-type name="value_type">
<apply-template name="std::iterator_traits">
<type name="Iter"/>
</apply-template>
</get-member-type>
<description><simpara>The value type of the iterator</simpara></description>
</associated-type>
<refines const="no" concept="ForwardIterator"/>
<notation variables="i j">
<sample-value>
<type name="Iter"/>
</sample-value>
</notation>
<associated-type name="category">
<get-member-type name="iterator_category">
<apply-template name="std::iterator_traits">
<type name="Iter"/>
</apply-template>
</get-member-type>
<description><simpara>The category of the iterator</simpara></description>
</associated-type>
<notation variables="x">
<sample-value>
<type name="value_type"/>
</sample-value>
</notation>
<valid-type-expression name="Category tag">
<description/>
<type name="category"/>
<return-type>
<derived-from testable="yes">
<type name="std::bidirectional_iterator_tag"/>
</derived-from>
</return-type>
</valid-type-expression>
<valid-expression name="Predecrement">
<predecrement>
<sample-value><reference-to><type name="Iter"/></reference-to></sample-value>
</predecrement>
<return-type>
<require-same-type testable="yes">
<reference-to><type name="Iter"/></reference-to>
</require-same-type>
</return-type>
<precondition><code>i</code> is incrementable (not
off-the-end) and some dereferenceable iterator <code>j</code> exists
such that <code>i == ++j</code></precondition>
</valid-expression>
<valid-expression name="Postdecrement">
<postdecrement>
<sample-value><reference-to><type name="Iter"/></reference-to></sample-value>
</postdecrement>
<return-type>
<require-same-type testable="yes"><type name="Iter"/></require-same-type>
</return-type>
<precondition>Same as for predecrement</precondition>
<semantics>Equivalent to <code>{Iter j = i; --i; return j;}</code></semantics>
<postcondition><code>i</code> is dereferenceable or
off-the-end</postcondition>
</valid-expression>
<complexity>
All iterator operations must take amortized constant time.
</complexity>
<invariant name="Predecrement must return object">
<code>&i = &(--i)</code>
</invariant>
<invariant name="Unique path through sequence">
<code>i == j</code> implies <code>--i == --j</code>
</invariant>
<invariant name="Increment and decrement are inverses">
<code>++i; --i;</code> and <code>--i; ++i;</code> must end up with the
value of <code>i</code> unmodified, if <code>i</code> both of the
operations in the pair are valid.
</invariant>
<example-model>
<pointer-to>
<type name="T"/>
</pointer-to>
</example-model>
<example-model>
<get-member-type name="iterator">
<apply-template name="std::list">
<type name="T"/>
</apply-template>
</get-member-type>
</example-model>
<see-also concept="RandomAccessIterator"/>
</concept>
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