c++ - How negate std::is_integral for use in tag dispatch? -

i've been playing around tag dispatch, , following code works expect:

#include <type_traits> #include <iostream>  void impl(std::true_type) { std::cout << "true\n"; } void impl(std::false_type) { std::cout << "false\n"; }  template<typename t> void dispatch(t&& val) {   impl(std::is_integral<typename std::remove_reference<t>::type>()); }  int main() {   dispatch(10);    // calls impl(std::true_type)   dispatch("");    // calls impl(std::false_type) } 

but if want negate condition, i'm running trouble. thought throw "!" code inside dispatch,

impl(!std::is_integral<t>());    // added "!" 

but won't compile.

what need code work?

you may instantiate std::integral_constant constexpr value this:

 impl(std::integral_constant<bool, !std::is_integral<t>::value>()); 

std::true_type , std::false_type aliases class. other way introduce metafunction this:

template <typename t> struct not_ : std::integral_constant<bool, !t::value> {}; 

and use (call) it:

impl(typename not_<std::is_integral<t>>::type()); 

or use similar boost::mpl