[boost] [range] SinglePassIterator concept check is too strict

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

[1] http://www.boost.org/doc/libs/1_48_0/libs/iterator/doc/new-iter-concepts.html#single-pass-iterators-lib-single-pass-iterators