Find a type in a parameter pack

c++, c++11

Solution

template <typename A, typename B, typename... C>
struct index_of
{
  static constexpr int const value =
    std::is_same<A, B>{}
    ? 0
    : (index_of<A, C...>::value >= 0) ? 1+index_of<A, C...>::value : -1;
};

template <typename A, typename B>
struct index_of<A, B>
{
  static constexpr int const value = std::is_same<A, B>{} -1;
};

Note the `std::is_same<A, B>{} -1` uses a conversion from `bool` to `int`.

Better by deriving from `integral_constant`:

template <typename A, typename B, typename... C>
struct index_of
  : std::integral_constant
    < int,
        std::is_same<A, B>{}
      ? 0
      : (index_of<A, C...>{} == -1 ? -1 : 1+index_of<A, C...>{})
    >
{};

template <typename A, typename B>
struct index_of<A, B>
  : std::integral_constant < int, std::is_same<A, B>{} -1 >
{};

If you don't need to return `-1` in case the type isn't found: (if anyone knows how to incorporate a `static_assert` here for a pretty diagnostic message, I'd appreciate a comment/edit)

template <typename A, typename B, typename... C>
struct index_of
  : std::integral_constant < std::size_t,
                             std::is_same<A, B>{} ? 0 : 1+index_of<A, C...>{} >
{};

template <typename A, typename B>
struct index_of<A, B>
  : std::integral_constant<std::size_t, 0>
{
    constexpr operator std::size_t() const
    {
        return   std::is_same<A, B>{}
               ? 0
               : throw std::invalid_argument("Type not found!");
    }
};

Problem

I'm trying to find type `A` in a parameter pack: ``` template <int I, typename A, typename B, typename ...C> struct index_of { static constexpr int const value = std::is_same<A, B>::value ? I : index_of<I + 1, A, C...>::value; }; template <int I, typename A, typename B> struct index_of<I, A, B> { static constexpr int const value = std::is_same<A, B>::value ? I : -1; }; ``` This seems to work, but I am unable to eliminate the nontype parameter `I`, which I would like to be a default parameter, but can't make it such, because of the parameter pack at the end. How to eliminate/hide `I`, so the metafunction becomes more user-friendly?

Original source