Template member function cast bug in VC80SP1 (and more...)

"Mycroft Holmes" <m.holmes@nospam.it>
Fri, 17 Aug 2007 11:50:19 +0200
Hi to all,

writing a small piece of metaprogramming code, we had to face some problems:
one at least seems a bug in VC8.0 (SP1).
here's an overview: we have

class VECTOR
class MAP_BASE : private VECTOR
class MAP : public MAP_BASE

MAP_BASE makes public some (originally public) things in VECTOR, such as
VECTOR::erase, and both MAP_BASE and MAP define 'iterator' as
we'll omit template parameters from the text, a precise c++ sample is
appended below.

(problem 1: less important)
if we write

typedef MAP::iterator (MAP::*pointer_to_erase_t)(MAP::iterator);
pointer_to_erase_t p = &MAP::erase

every compiler we tried (vc8, gcc4, intel10) says that 'erase' is
inaccessible (in a private base), despite the fact that the member is...
public, due to the using declaration.
actually we had a quick look in the standard, and surprisingly, it just says
'if the member is in a private base, the conversion (of the pointer) fails.
is this an omission?

(problem 2: very important)

is there some way to test if a class has ANY member/member function
named -let's say- swap?
we just need to know if the expression "&T::swap" is legal or not.
first, we tried the following but it seems that it's illegal:

template <typename S>
class has_something_named_swap
 class yes { char dummy[2]; };
 typedef char no;

 template <typename T>
  static yes test(T*, size_t = sizeof(&T::swap));

 static no test(void*, size_t = 0);


 static const bool value = sizeof(test( (S*)0 ))!= sizeof(no);

the following variant works well in gcc4 and intel10 (and according to the
SFINAE principle, it should work!!!)
but does not compile in vc8 (it says something like: overloaded function,
illegal sizeof operand):

 template <size_t N> class yes { char dummy[2]; };

 template <typename T>
  static yes<sizeof(&T::swap)> test(T*);

I personally think it's an artificial limitation in VC.

(problem 3: very important)
it looks like VC8 is unable to resolve a
pointer-to-overloaded-member-function when some of the overloads are in
VECTOR and some are in MAP_BASE.
Here's a minimal sample that shows the problem: both intel10 and gcc4 do
compile it correctly.

the most satisfactory workaround we found is to duplicate VECTOR::erase in
MAP_BASE (it's commented in the code), but as a rule this is not nice (if
also std::map had the same problem, we would have had to overload erase_gap
on std::map, which is exactly what we are trying to avoid...)

#include <map>
#include <vector>

template <typename T>
class VECTOR
 typedef std::vector<T> vector_t;
 vector_t data_;


 typedef typename vector_t::iterator iterator;

 iterator begin() { return data_.begin(); }

 iterator erase(iterator pos)
  return (pos);

template <typename key_t, typename mapped_t>
class MAP_BASE : private VECTOR< std::pair<key_t, mapped_t> >

 typedef VECTOR< std::pair<key_t, mapped_t> > map_t;

 typedef typename map_t::iterator iterator;

 using map_t::begin;
 using map_t::erase;

 size_t erase(const key_t& x)
  return 0;

// iterator erase(iterator pos)
// {
// return map_t::erase(pos);
// }


template <typename key_t, typename mapped_t>
class MAP : public MAP_BASE<key_t, mapped_t>
 typedef MAP_BASE<key_t, mapped_t> base_t;


  using base_t::iterator;

template <typename container_t, typename iterator_t, typename base_t>
inline void erase_gap2(container_t& c, iterator_t& i, iterator_t
 i = c.erase(i);

template <typename container_t, typename iterator_t, typename base_t>
inline void erase_gap2(container_t& c, iterator_t& i, void

template <typename container_t>
typename container_t::iterator erase_gap(container_t& c, typename
container_t::iterator i)
 erase_gap2(c, i, &container_t::erase);
 return i;

int main()
 std::vector<double> v;
 std::map<double, double> m;
  MAP<double, double> w;

 erase_gap(w, w.begin());
 erase_gap(v, v.begin());
 erase_gap(m, m.begin());

 return 0;

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