From: Lee Houghton (lee_at_[hidden])
Date: 2006-01-26 09:47:18
Note: I'm using boost 1.33.1 here
Sebastian Redl wrote:
> This code should really not give you any warnings. (And doesn't on
GCC, though I doubt that'll help you.)
> C4244, so that others don't have to look it up, is the "conversion
with possible data loss" warning. This indicates that in VC++,
make_counting_iterator returns a counting_iterator<long> or even
<__int64>. Can somebody with that compiler post the output of this line?
> std::cout << typeid(boost::make_counting_iterator(1)).name() <<
std::endl;
Using Visual Studio .NET 2003, the output of this was:
class boost::counting_iterator<int,struct boost::use_default,struct
boost::use_default>
as it should be.
> A simple workaround for this particular warning ought to be to
directly instantiate counting_iterators with their template parameter
set correctly.
>
> Sebastian Redl
The problem appears to be when operator += is called on the iterator.
Here, the difference_type of the iterator,
boost::iterator_facade<Derived,Value,CategoryOrTraversal,Reference,Difference>::difference_type,
which is equal to
boost::detail::counting_iterator_base<int,boost::use_default,boost::use_default>::difference,
is __int64. The counting_iterator_base uses the following typedef:
typedef typename detail::ia_dflt_help<
Difference
, mpl::eval_if<
is_numeric<Incrementable>
, numeric_difference<Incrementable>
, iterator_difference<Incrementable>
>
>::type difference;
It seems this has been discovered before:
http://listarchives.boost.org/Archives/boost/2004/07/69347.php
The actual warnings I received compiling this code were:
--------------------------------------------------------
Compiling...
BoostTest1.cpp
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\xutility(481) : warning C4244: '+=' : conversion from
'boost::iterators::enabled<__formal>::base<T>::type' to
'std::vector<_Ty>::size_type', possible loss of data
with
[
__formal=true,
T=std::iterator_traits<boost::counting_iterator<int,boost::use_default,boost::use_default>>::difference_type
]
and
[
_Ty=int
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\xutility(498) : see reference to function template
instantiation 'void std::_Distance2<_InIt,_Diff>(_RanIt,_RanIt,_Diff
&,std::random_access_iterator_tag)' being compiled
with
[
_InIt=boost::counting_iterator<int,boost::use_default,boost::use_default>,
_Diff=std::vector<int>::size_type,
_RanIt=boost::counting_iterator<int,boost::use_default,boost::use_default>
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\vector(657) : see reference to function template
instantiation 'void
std::_Distance<_Iter,std::vector<_Ty>::size_type>(_InIt,_InIt,_Diff &)'
being compiled
with
[
_Iter=boost::counting_iterator<int,boost::use_default,boost::use_default>,
_Ty=int,
_InIt=boost::counting_iterator<int,boost::use_default,boost::use_default>,
_Diff=std::vector<int>::size_type
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\vector(634) : see reference to function template
instantiation 'void
std::vector<_Ty>::_Insert<_Iter>(std::vector<_Ty>::iterator,_Iter,_Iter,std::forward_iterator_tag)'
being compiled
with
[
_Ty=int,
_Iter=boost::counting_iterator<int,boost::use_default,boost::use_default>
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\vector(366) : see reference to function template
instantiation 'void
std::vector<_Ty>::insert<_Iter>(std::vector<_Ty>::iterator,_Iter,_Iter)'
being compiled
with
[
_Ty=int,
_Iter=boost::counting_iterator<int,boost::use_default,boost::use_default>
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\vector(343) : see reference to function template
instantiation 'void
std::vector<_Ty>::_Construct<_Iter>(_Iter,_Iter,std::input_iterator_tag)'
being compiled
with
[
_Ty=int,
_Iter=boost::counting_iterator<int,boost::use_default,boost::use_default>
]
g:\Projects\BoostTest1\BoostTest1.cpp(15) : see reference to
function template instantiation
'std::vector<_Ty>::vector<boost::counting_iterator<Incrementable,CategoryOrTraversal,Difference>>(_Iter,_Iter)'
being compiled
with
[
_Ty=int,
Incrementable=int,
CategoryOrTraversal=boost::use_default,
Difference=boost::use_default,
_Iter
=boost::counting_iterator<int,boost::use_default,boost::use_default>
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\boost\iterator\iterator_adaptor.hpp(329) : warning
C4244: '+=' : conversion from
'boost::iterator_facade<Derived,Value,CategoryOrTraversal,Reference,Difference>::difference_type'
to 'int', possible loss of data
with
[
Derived=boost::counting_iterator<int,boost::use_default,boost::use_default>,
Value=const boost::mpl::identity<const int>::type,
CategoryOrTraversal=boost::mpl::identity<boost::random_access_traversal_tag>::type,
Reference=boost::mpl::identity<const int &>::type,
Difference=boost::mpl::identity<boost::detail::counting_iterator_base<int,boost::use_default,boost::use_default>::difference>::type
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\boost\iterator\iterator_adaptor.hpp(327) : while
compiling class-template member function 'void
boost::iterator_adaptor<Derived,Base,Value,Traversal,Reference,Difference>::advance(boost::iterator_facade<Derived,Value,CategoryOrTraversal,Reference,Difference>::difference_type)'
with
[
Derived=boost::counting_iterator<int,boost::use_default,boost::use_default>,
Base=int,
Value=const int,
Traversal=boost::detail::counting_iterator_base<int,boost::use_default,boost::use_default>::traversal,
Reference=const int &,
Difference=boost::detail::counting_iterator_base<int,boost::use_default,boost::use_default>::difference,
CategoryOrTraversal=boost::mpl::identity<boost::random_access_traversal_tag>::type
]
d:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\boost\iterator\counting_iterator.hpp(153) : see
reference to class template instantiation
'boost::iterator_adaptor<Derived,Base,Value,Traversal,Reference,Difference>'
being compiled
with
[
Derived=boost::counting_iterator<int,boost::use_default,boost::use_default>,
Base=int,
Val
ue=const int,
Traversal=boost::detail::counting_iterator_base<int,boost::use_default,boost::use_default>::traversal,
Reference=const int &,
Difference=boost::detail::counting_iterator_base<int,boost::use_default,boost::use_default>::difference
]
g:\Projects\BoostTest1\BoostTest1.cpp(12) : see reference to
class template instantiation
'boost::counting_iterator<Incrementable,CategoryOrTraversal,Difference>'
being compiled
with
[
Incrementable=int,
CategoryOrTraversal=boost::use_default,
Difference=boost::use_default
]