非类型可变参数模板的运行时计算

Question

Is it possible to make run time calculations with non-type variadic templates?

Imagine the following situation:

template<unsigned int... indexes>
struct s
{
   unsigned int getPosition(const unsigned int target) const;
};

s<2,5,0,7> obj;
std::cout << obj.getPosition(5) << std::endl; //< this should output 1
std::cout << obj.getPosition(2) << std::endl; //< this should output 0

The getPosition method should return the position of the given integer in the template parameter pack. If the given integer is not present in the parameter pack, an error (preferably at compile time) should be generated.

PS I know how to do this if the signature is changed to

template<unsigned int... indexes>
struct s
{
   template<unsigned int target>
   unsigned int getPosition() const;
};

Unfortunately, this is not an option for me as the method should be virtual.

Answer 1

You could dump the indexes into a std::array and use std::find .

struct s
{
   unsigned int getPosition(const unsigned int target) const {
       static std::array<int, sizeof...(indexes)> indexArray { indexes... };
       auto pos = std::find(std::begin(indexArray), std::end(indexArray), target);

       if (pos == std::end(indexArray)) {
           //handle error, probably an exception   
       }

       return std::distance(std::begin(indexArray), pos);
   }
};

You can't generate a compile-time error if target doesn't exist, as you only know target at run-time.