Efficient variadic equals-any function

Question

I want to make a function that returns whether the first parameter equals any of the others. This is what I came up with:

template<typename T, typename... U>
bool equalsAny(T first, U... args) {
    U arr[] {args...};
    for (auto& arg : arr) if (first == arg) return true;
    return false;
}

This is of course very slow in the sense that an array must be created for all arguments. Is there any way to make this almost as fast as first == arg1 || first == arg2 || ... by for instance using c++17 fold expressions?

Answer 1

You tagged C++17 so... what's wrong with template folding ?

template <typename T, typename... U>
bool equalsAny (T first, U... args)
 { return (first == args || ...); }

Pre C++17, you can use the initialization of an unused C-style array (I'm not an expert but I suppose that a good compiler can optimize it very well)

template <typename T, typename... U>
bool equalsAny (T first, U... args)
 { 
   using unused = bool[];

   bool ret { false };

   (void)unused { false, (ret = ret || (first == args))... };

   return ret;
}