Re: projection_iterator and ptr_fun

From: rogeeff (rogeeff_at_[hidden])
Date: 2002-02-11 20:33:48

--- In boost_at_y..., "David Abrahams" <david.abrahams_at_r...> wrote:
> Support requests 101: always include a description of the problem 
you're
> experiencing.
> "It doesn't work" is not sufficiently specific to allow anyone to 
help you.
> 

I intentionally did not include compilation error, since it could be 
misdirecting. Well here an error prodused by MSVC with STLPort:

....\stlport-4.5\stlport\stl\_algobase.h(111) : error C2679: 
binary '=' : no operator defined which takes a right-hand operand of 
type 'const class _STL::basic_string<char,class 
_STL::char_traits<char>,class _STL::allocator<char> > *' (or 
there is no acceptable conversion)
.....\stlport-4.5\stlport\stl\_algobase.h(210) : see reference to 
function template instantiation 'class _STL::ostream_iterator<class 
_STL::basic_string<char,class _STL::char_traits<char>,class 
_STL::allocator<char> >,char,class _ST
L::char_traits<char> > __cdecl _STL::__copy(struct 
boost::iterator_adaptor<struct _STL::_List_iterator<struct 
personnel_record,struct _STL::_Nonconst_traits<struct 
personnel_record> >,struct boost::projection_iterator_policies<class 
_STL::pointer_to
_unary_function<struct personnel_record const &,class 
_STL::basic_string<char,class _STL::char_traits<char>,class 
_STL::allocator<char> > const &> >,class 
_STL::basic_string<char,class _STL::char_traits<char>,class 
_STL::allocator<char> > const &,cl
ass _STL::basic_string<char,class _STL::char_traits<char>,class 
_STL::allocator<char> > const * &,class _STL::basic_string<char,class 
_STL::char_traits<char>,class _STL::allocator<char> > const * 
*,struct boost::detail::default_argument,struct boost
::detail::default_argument>,struct boost::iterator_adaptor<struct 
_STL::_List_iterator<struct personnel_record,struct 
_STL::_Nonconst_traits<struct personnel_record> >,struct 
boost::projection_iterator_policies<class 
_STL::pointer_to_unary_function<
struct personnel_record const &,class _STL::basic_string<char,class 
_STL::char_traits<char>,class _STL::allocator<char> > const &> 
>,class _STL::basic_string<char,class _STL::char_traits<char>,class 
_STL::allocator<char> > const &,class _STL::basic_
string<char,class _STL::char_traits<char>,class _STL::allocator<char> 
> const * &,class _STL::basic_string<char,class 
_STL::char_traits<char>,class _STL::allocator<char> > const * 
*,struct boost::detail::default_argument,struct boost::detail::defaul
t_argument>,class _STL::ostream_iterator<class 
_STL::basic_string<char,class _STL::char_traits<char>,class 
_STL::allocator<char> >,char,class _STL::char_traits<char> >,const 
struct _STL::input_iterator_tag &,int *)' being compiled

///////////////////////////////////////////////////////////////////
// From STL Port implementation:

template <class _InputIter, class _OutputIter, class _Distance>
inline _OutputIter __copy(_InputIter __first, _InputIter __last,
                          _OutputIter __result,
                          const input_iterator_tag &, _Distance*) {
  for ( ; __first != __last; ++__result, ++__first)
error is here -> *__result = *__first;
  return __result;
}

//////////////////////////////////////////////////////////////////

This error basicly means that iterator_aadaptor for some (magic) 
resons deduced that resulting value type is std::string const* while 
it should be std::string const.
  Other compilers give me other errors. It seems that the root 
problem is that pointer_to_unary function incorrectly define 
result_type. While all others functors that returns reference (like 
identity, select1st,select2nd ) define result_type as T if operator() 
returns T const&, pointer_to_unary_function automatically propogate 
result type inherited from its argument, i.e. result_type is defined 
like T const&. Could we use transforming type_traits here and define 
more smart ptr_fun or I just missing something?

Gennadiy.