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.
The new implementation tries to detect if the incremented/decremented type
is an iterator first and if not falls back to operator probing. This way
iterators that are not SFINAE-friendly (i.e. unconditionally define
arithmetic operators regardless of the iterator category) are still treated
as iterators through std::advance and do not fail the compilation.
The iterator detection is based on probing for the nested iterator_category
type that is expected to be present in class-type iterators. This heuristic
is not flawless since iterators are not required to defined this type.
User-defined iterators may not have it and instead specialize
std::iterator_traits. This use case is not covered by the current implementation
and will likely fail to compile. With C++17 SFINAE-friendly std::iterator_traits
this can be fixed, but currently Boost.Config lacks the macro to detect
availability of this feature. Support for it can be added by a later commit.
Also simplified boost::prior for iterators, removing the possibility of
integer overflow caused by negation of the distance value.
VS2013, VS2015 & VS2017RC don't like the ternary throwing an exception :
'return': cannot convert from 'void' to 'const char &'
Now using classic if when compiling on a windows platform.
All that boost/iterator.hpp does is pull std::iterator into namespace boost. A comment in that header mentions: "This header is obsolete and will be deprecated."
The new version should provide the expected behavior in the case (prior(v.end(), v.size()) == v.begin()). It should also work with integers now, as was originally intended by David Abrahams. Added tests to verify these new use cases.
Gcc 4.5 does not allow non-public defaulted functions, so fall back to the C++03 version. Also replaced the deprecated macros with the new ones and adjusted formatting.
