cpp-variant/include/variant/detail/types.hpp

#pragma once

namespace variant { namespace detail {

  // Max of a list of values
  template <size_t T, size_t... Ts> struct static_max;

  template <size_t T> struct static_max<T> {
    static const constexpr size_t value = T;
  };

  template <size_t T1, size_t T2, size_t... Ts>
  struct static_max<T1, T2, Ts...> {
    static const constexpr size_t value =
        T1 > T2 ? static_max<T1, Ts...>::value : static_max<T2, Ts...>::value;
  };

  // Given a typelist of size N, type_index<F>::value will return the inverted
  // offset (i.e. the number of elements that remain after F in the typelist).
  template <typename F, typename... Ts> struct type_index;

  template <typename F> struct type_index<F> {};

  template <typename F, typename T, typename... Ts>
  struct type_index<F, T, Ts...> {
    static const constexpr size_t value = type_index<F, Ts...>::value;
  };

  template <typename F, typename... Ts> struct type_index<F, F, Ts...> {
    static const constexpr size_t value = sizeof...(Ts);
  };

}}