First of all, I understand that this is a requirement imposed by the Standard.
However, the requirement, in general, is valid only for iterators that do not
refer to any container. This is not the case with iterators useful in
algorithms; the default value of such an iterator has no meaning.
Moreover, the implementation of the library has no obligation to check this
trait for every algorithm, and I suggest that this check should be removed.
The code for std::lower_bound does not use this feature in a meaningful way,
and it is straightforward to fix.
The patch below, apart from fixing the problem, also replaces iterator with
std::iterator.
template<typename _ForwardIterator, typename _Tp>
_ForwardIterator
lower_bound(_ForwardIterator __first, _ForwardIterator __last,
const _Tp& __val)
{
typedef typename std:: iterator_traits<_ForwardIterator>::value_type
_ValueType;
typedef typename std:: iterator_traits<_ForwardIterator>::difference_type
_DistanceType;
// concept requirements
__glibcxx_function_requires(_ForwardIteratorConcept<_ForwardIterator>)
__glibcxx_function_requires(_LessThanOpConcept<_ValueType, _Tp>)
__glibcxx_requires_partitioned_lower(__first, __last, __val);
_DistanceType __len = std:: distance(__first, __last);
_DistanceType __half;
while (__len > 0)
{
_ForwardIterator __middle ((__first));
__half = __len >> 1;
std::advance(__middle, __half);
if (*__middle < __val)
{
__first = __middle;
++__first;
__len = __len - __half - 1;
}
else
__len = __half;
}
return __first;
}
--
Summary: lower_bound iterator must be default constructible
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: giecrilj at stegny dot 2a dot pl
GCC build triplet: x86_64-suse-linux
GCC host triplet: x86_64-suse-linux
GCC target triplet: x86_64-suse-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45488
