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< title > Bidirectional Traversal Concept</ title >
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< h1 class = "title" > Bidirectional Traversal Concept</ h1 >
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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
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< p > A class or built-in type < tt class = "docutils literal" >< span class = "pre" > X</ span ></ tt > models the < em > Bidirectional Traversal</ em >
concept if, in addition to < tt class = "docutils literal" >< span class = "pre" > X</ span ></ tt > meeting the requirements of Forward
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Traversal Iterator, the following expressions are valid and respect
the stated semantics.</ p >
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< table border = "1" class = "docutils" >
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< colgroup >
< col width = "38%" />
< col width = "37%" />
< col width = "25%" />
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< thead valign = "bottom" >
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< tr >< th class = "head" colspan = "3" > Bidirectional Traversal Iterator Requirements (in addition to Forward Traversal
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Iterator)</ th >
</ tr >
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< tr >< th class = "head" > Expression</ th >
< th class = "head" > Return Type</ th >
< th class = "head" > Assertion/Semantics /
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Pre-/Post-condition</ th >
</ tr >
</ thead >
< tbody valign = "top" >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > --r</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > X& </ span ></ tt ></ td >
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< td > pre: there exists
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< tt class = "docutils literal" >< span class = "pre" > s</ span ></ tt > such that < tt class = "docutils literal" >< span class = "pre" > r</ span >
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< span class = "pre" > ==</ span > < span class = "pre" > ++s</ span ></ tt > . post:
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< tt class = "docutils literal" >< span class = "pre" > s</ span ></ tt > is
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dereferenceable.
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< tt class = "docutils literal" >< span class = "pre" > --(++r)</ span > < span class = "pre" > ==</ span > < span class = "pre" > r</ span ></ tt > .
< tt class = "docutils literal" >< span class = "pre" > --r</ span > < span class = "pre" > ==</ span > < span class = "pre" > --s</ span ></ tt >
implies < tt class = "docutils literal" >< span class = "pre" > r</ span > < span class = "pre" > ==</ span >
< span class = "pre" > s</ span ></ tt > . < tt class = "docutils literal" >< span class = "pre" > & r</ span > < span class = "pre" > ==</ span > < span class = "pre" > & --r</ span ></ tt > .</ td >
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</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > r--</ span ></ tt ></ td >
< td > convertible to < tt class = "docutils literal" >< span class = "pre" > const</ span > < span class = "pre" > X& </ span ></ tt ></ td >
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< td >< pre class = "first last literal-block" >
{
X tmp = r;
--r;
return tmp;
}
</ pre >
</ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > iterator_traversal< X> ::type</ span ></ tt ></ td >
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< td > Convertible to
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< tt class = "docutils literal" >< span class = "pre" > bidirectional_traversal_tag</ span ></ tt ></ td >
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< td > </ td >
</ tr >
</ tbody >
</ table >
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