Re: [Boost-users] enable_if and templated operator() of member of class template

1 Mar 2012

      "MM"  writes:
...
Hello,
I am using the enable_if construct to disable an operator() template member
function of a templated variant visitor:
template <typename iter_t>
class iterator_compare_visitor_ : public boost::static_visitor<bool> {
public:
  iterator_compare_visitor_(const iter_t& to_compare_to)
    : to_compare_to_(to_compare_to)
  {}
template <typename variant_iter_t>
  typename std::enable_if< boost::mpl::contains::value, bool >::type
  operator()(const variant_iter_t& it) const
  {
    return std::is_same::value &&
it==to_compare_to_;
  }
template <typename variant_iter_t> 
  typename std::enable_if< boost::mpl::contains::value, bool >::type 
  operator()(const variant_iter_t& it) const
  {
    return std::is_same::value &&
it==to_compare_to_.base();
  }
private:
  const iter_t& to_compare_to_;
};
The compilation fails in msvc2010 with:
error C2039: 'type' : is not a member of 'std::tr1::enable_if<_Test,_Type>'
1>          with
1>          [
1>              _Test=false,
1>              _Type=bool
1>          ]
in this case, iter_t is not contained in const_reverse_iterators_t and so
the 2nd operator() should not be enabled.
Rds,
MM
What you are trying cannot work as this is not a valid way to overload
operator(). Consider using enable_if as an argument to your functions:

http://codepad.org/nduBfhT6

As for the actual error: It looks like your compiler is using
std::tr1::enable_if even if you specify `std::enable_if`. Those should
be equivalent. You might want to use boost::enable_if.

Also, please try to provide snippets that don't require modifications to
show the bug (e.g. includes).

HTH,
Philipp Moeller