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< title > Random Access Traversal Concept</ title >
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< h1 class = "title" > Random Access Traversal Concept</ h1 >
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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
< p > A class or built-in type < tt class = "docutils literal" >< span class = "pre" > X</ span ></ tt > models the < em > Random Access Traversal</ em >
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concept if the following expressions are valid and respect the stated
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semantics. In the table below, < tt class = "docutils literal" >< span class = "pre" > Distance</ span ></ tt > is
< tt class = "docutils literal" >< span class = "pre" > iterator_traits< X> ::difference_type</ span ></ tt > and < tt class = "docutils literal" >< span class = "pre" > n</ span ></ tt > represents a
constant object of type < tt class = "docutils literal" >< span class = "pre" > Distance</ span ></ tt > .</ p >
< table border = "1" class = "docutils" >
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< colgroup >
< col width = "28%" />
< col width = "30%" />
< col width = "23%" />
< col width = "20%" />
</ colgroup >
< thead valign = "bottom" >
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< tr >< th class = "head" colspan = "4" > Random Access Traversal Iterator Requirements (in addition to Bidirectional Traversal)</ th >
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</ tr >
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< tr >< th class = "head" > Expression</ th >
< th class = "head" > Return Type</ th >
< th class = "head" > Operational Semantics</ th >
< th class = "head" > Assertion/
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Precondition</ th >
</ tr >
</ thead >
< tbody valign = "top" >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > r</ span > < span class = "pre" > +=</ span > < span class = "pre" > n</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > X& </ span ></ tt ></ td >
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< td >< pre class = "first last literal-block" >
{
Distance m = n;
if (m > = 0)
while (m--)
++r;
else
while (m++)
--r;
return r;
}
</ pre >
</ td >
< td > </ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > +</ span > < span class = "pre" > n</ span ></ tt > , < tt class = "docutils literal" >< span class = "pre" > n</ span > < span class = "pre" > +</ span > < span class = "pre" > a</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > X</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > {</ span > < span class = "pre" > X</ span > < span class = "pre" > tmp</ span > < span class = "pre" > =</ span > < span class = "pre" > a;</ span > < span class = "pre" > return</ span > < span class = "pre" > tmp</ span >
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< span class = "pre" > +=</ span > < span class = "pre" > n;</ span > < span class = "pre" > }</ span ></ tt ></ td >
< td > </ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > r</ span > < span class = "pre" > -=</ span > < span class = "pre" > n</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > X& </ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > return</ span > < span class = "pre" > r</ span > < span class = "pre" > +=</ span > < span class = "pre" > -n</ span ></ tt ></ td >
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< td > </ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > -</ span > < span class = "pre" > n</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > X</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > {</ span > < span class = "pre" > X</ span > < span class = "pre" > tmp</ span > < span class = "pre" > =</ span > < span class = "pre" > a;</ span > < span class = "pre" > return</ span > < span class = "pre" > tmp</ span >
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< span class = "pre" > -=</ span > < span class = "pre" > n;</ span > < span class = "pre" > }</ span ></ tt ></ td >
< td > </ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > b</ span > < span class = "pre" > -</ span > < span class = "pre" > a</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > Distance</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > < </ span > < span class = "pre" > b</ span > < span class = "pre" > ?</ span > < span class = "pre" > distance(a,b)</ span >
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< span class = "pre" > :</ span > < span class = "pre" > -distance(b,a)</ span ></ tt ></ td >
< td > pre: there exists a
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value < tt class = "docutils literal" >< span class = "pre" > n</ span ></ tt > of
< tt class = "docutils literal" >< span class = "pre" > Distance</ span ></ tt > such that
< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > +</ span > < span class = "pre" > n</ span > < span class = "pre" > ==</ span > < span class = "pre" > b</ span ></ tt > . < tt class = "docutils literal" >< span class = "pre" > b</ span >
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< span class = "pre" > ==</ span > < span class = "pre" > a</ span > < span class = "pre" > +</ span > < span class = "pre" > (b</ span > < span class = "pre" > -</ span > < span class = "pre" > a)</ span ></ tt > .</ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a[n]</ span ></ tt ></ td >
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< td > convertible to T</ td >
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< td >< tt class = "docutils literal" >< span class = "pre" > *(a</ span > < span class = "pre" > +</ span > < span class = "pre" > n)</ span ></ tt ></ td >
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< td > pre: a is a < em > Readable
Iterator</ em ></ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a[n]</ span > < span class = "pre" > =</ span > < span class = "pre" > v</ span ></ tt ></ td >
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< td > convertible to T</ td >
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< td >< tt class = "docutils literal" >< span class = "pre" > *(a</ span > < span class = "pre" > +</ span > < span class = "pre" > n)</ span > < span class = "pre" > =</ span > < span class = "pre" > v</ span ></ tt ></ td >
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< td > pre: a is a < em > Writable
iterator</ em ></ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > < </ span > < span class = "pre" > b</ span ></ tt ></ td >
< td > convertible to < tt class = "docutils literal" >< span class = "pre" > bool</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > b</ span > < span class = "pre" > -</ span > < span class = "pre" > a</ span > < span class = "pre" > > </ span > < span class = "pre" > 0</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > < </ span ></ tt > is a total
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ordering relation</ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > > </ span > < span class = "pre" > b</ span ></ tt ></ td >
< td > convertible to < tt class = "docutils literal" >< span class = "pre" > bool</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > b</ span > < span class = "pre" > < </ span > < span class = "pre" > a</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > > </ span ></ tt > is a total
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ordering relation</ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > > =</ span > < span class = "pre" > b</ span ></ tt ></ td >
< td > convertible to < tt class = "docutils literal" >< span class = "pre" > bool</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > !(a</ span > < span class = "pre" > < </ span > < span class = "pre" > b)</ span ></ tt ></ td >
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< td > </ td >
</ tr >
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< tr >< td >< tt class = "docutils literal" >< span class = "pre" > a</ span > < span class = "pre" > < =</ span > < span class = "pre" > b</ span ></ tt ></ td >
< td > convertible to < tt class = "docutils literal" >< span class = "pre" > bool</ span ></ tt ></ td >
< td >< tt class = "docutils literal" >< span class = "pre" > !(a</ span > < span class = "pre" > > </ span > < span class = "pre" > b)</ span ></ tt ></ td >
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< td > </ 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" > random_access_traversal_tag</ span ></ tt ></ td >
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< td > </ td >
< td > </ td >
</ tr >
</ tbody >
</ table >
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