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Use Boost.Iterator to advance iterators.

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By using Boost.Iterator we rely on the separate traversal category instead of
the standard iterator category to advance iterators efficiently. For instance,
this allows to advance transform iterators over a random access sequence
in constant time, despite that they are formally input iterators.

Also, std::reverse_iterator formally requires at least bidirectional iterator
as the underlying iterator type. Transform iterators from the example above
don't qualify, so potentially std::reverse_iterator could fail to compile.
This commit is contained in:
Andrey Semashev 2017-07-12 21:08:13 +03:00
parent 5bc9e47688
commit e25d85446e
1 changed files with 5 additions and 3 deletions
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8
include/boost/next_prior.hpp
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@ -22,6 +22,8 @@
#include <boost/type_traits/has_plus_assign.hpp> #include <boost/type_traits/has_plus_assign.hpp>
#include <boost/type_traits/has_minus.hpp> #include <boost/type_traits/has_minus.hpp>
#include <boost/type_traits/has_minus_assign.hpp> #include <boost/type_traits/has_minus_assign.hpp>
#include <boost/iterator/advance.hpp>
#include <boost/iterator/reverse_iterator.hpp>
namespace boost { namespace boost {
@ -101,7 +103,7 @@ struct next_advance_impl< T, Distance, true >
{ {
static T call(T x, Distance n) static T call(T x, Distance n)
{ {
std::advance(x, n); boost::iterators::advance(x, n);
return x; return x;
} }
}; };
@ -147,8 +149,8 @@ struct prior_advance_impl< T, Distance, true >
static T call(T x, Distance n) static T call(T x, Distance n)
{ {
// Avoid negating n to sidestep possible integer overflow // Avoid negating n to sidestep possible integer overflow
std::reverse_iterator< T > rx(x); boost::iterators::reverse_iterator< T > rx(x);
std::advance(rx, n); boost::iterators::advance(rx, n);
return rx.base(); return rx.base();
} }
}; };