Subject: [boost] [range] SinglePassIterator concept check is too strict
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-01-09 21:22:09
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.)
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?
Regards,
Nate