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

On Thu, 1 Mar 2012, MM wrote:

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> ]

The main issue here is that the condition in enable_if does not depend on the template parameter to operator(); see the version in http://codepad.org/6AORo7tU, in which I've used boost::enable_if and added some tests. One fix is to make the enable_if condition dependent, as in http://codepad.org/916Tl6W8 (there were a number of other issues in the code; note that the is_same condition needs to be in the enable_if since the body of the function will not always compile when that is false). Please see if that last paste works for you. -- Jeremiah Willcock