on Mon Jan 09 2012, Nathan Ridge <zeratul976-AT-hotmail.com> wrote:

Hello,

It seems to me the concept checking performed by Boost.Range for the SinglePassIterator concept is too strict.

In boost/range/concepts.hpp, SinglePassIteratorConcept checks for the following usage (line 157 in trunk):

BOOST_DEDUCED_TYPENAME boost::detail::iterator_traits<Iterator>::reference r1(*i);

This requires that the return type of operator* be convertible to the iterator's reference type.

However, looking at the specification for single pass iterators [1], I don't see any requirement that the return type of operator* be convertible to the reference type - it just needs to be convertible to the value type.

An example of an iterator for which the return type of operator* is not convertible to the iterator's reference type is std::istreambuf_iterator<charT>, whose reference type is charT&, but the return type of its operator* is charT. (Both of these are specified explicitly in section 24.5.3 of the C++03 standard.)

I agree with your analysis.

As a result, code such as the following fails to compile with GCC or MSVC in C++03 mode:

#include <boost/range/concepts.hpp> #include <boost/range/iterator_range.hpp> #include <iterator> #include <istream>

int main() {     BOOST_CONCEPT_ASSERT     ((         boost::SinglePassRangeConcept<             boost::iterator_range<                 std::istreambuf_iterator<char> > >     )); }

(In C++11, things changed - a requirement was added that the return type

of operator* be the same as the iterator's reference type, and

correspondingly std::istreambuf_iterator<charT>'s reference type was

changed to be charT).

What can be done about this?

The code could be fixed? Submit a patch I suppose. -- Dave Abrahams BoostPro Computing http://www.boostpro.com