helper function to check a value is any of its parameters

Question

I need to check if a value of type T is any of its parameters using a helper function.

For example, using something like the following code

enum class my_enum{k1, k2, k3, k4, k5};
auto v{my_enum::k1};
if (is_any_of(v, my_enum::k1, my_enum::k4, my_enum::k5)) {
}

rather than using if (v == my_enum::k1 || v == my_enum::k4 || v== my_enum::k5) {} or using switch-case.

How do I implement the variadic function bool is_any_of() in C++11?

How the implementation would become simpler with C++17 fold expressions?

Answer 1

This will work in C++11 and will do what you want as long as all of the types can be compared to each other.

template<typename T, typename R>
bool is_any_of(T t, R r)
{
   return t == r;
}

template<typename T, typename R, typename... ARGS>
bool is_any_of(T t, R r, ARGS... args)
{
   if (t == r)
   {
      return true;
   }
   else
   {
      return is_any_of(t, args...);
   }
}

This is much more compact and will work in C++17

template<typename T, typename... ARGS>
bool is_any_of(T t, ARGS... args)
{
   return ((t == args) || ...);
}