FeatureHelper.hpp

#include <cstdint>
#include <type_traits>
#include <utility>

/// Specialize the T<true, n> base classes to deduce inheritance based on value, where n is a bit position.
/// i.e:
///     class Base : FeatureHelper<T, 5u>{};
///     ==
///     class Base : T<true, 0>, T<true, 2>{};
///
/// Example:
/// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
/// struct X1 {};
/// struct X2 {};
/// struct X5 {};
///
/// template<bool, std::size_t> struct Feature {};
/// template<> struct Feature<true, 0> : X1 {};
/// template<> struct Feature<true, 1> : X2 {};
/// template<> struct Feature<true, 4> : X5 {};
///
/// template<uint8_t v>
/// class Base : FeatureHelper<Feature, v> {};
///
/// int main() {
///     static_assert(std::is_base_of_v<X1, Base<19u>>);
///     static_assert(std::is_base_of_v<X2, Base<19u>>);
///     static_assert(std::is_base_of_v<X5, Base<19u>>);
/// }
/// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
template<template<bool, std::size_t> typename T, auto v, typename = std::make_index_sequence<sizeof(v) * 8u>>
struct FeatureHelper;

template<template<bool, std::size_t> typename T, auto v, std::size_t... Is>
struct FeatureHelper<T, v, std::index_sequence<Is...>> : public T<(v & (1 << Is)) != 0u, Is>... {};