typelist.h

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#pragma once
 
#include <concepts>
#include <cstddef>
#include <utility>
#include <variant>
 
namespace osc
{
    // inspired by: https://codereview.stackexchange.com/questions/269320/c17-typelist-manipulation
    //
    // ... which was inspired by the book: "Modern C++ Design" (A. Alexandrescu, 2002)
 
    template<typename...>
    struct Typelist;
 
    template<>
    struct Typelist<> {};
 
    template<typename Head, typename... Tails>
    struct Typelist<Head, Tails...> {
        using head = Head;
        using tails = Typelist<Tails...>;
 
        template<typename T>
        static constexpr bool contains() noexcept
        {
            const auto f = []<typename... Ts>(Typelist<Ts...>)
            {
                return (std::same_as<T, Ts> or ...);
            };
            return f(Typelist{});
        }
    };
 
    // Typelist: size (the number of types held within)
 
    template<typename TList>
    struct TypelistSize;
 
    template<typename... Types>
    struct TypelistSize<Typelist<Types...>> {
        static constexpr size_t value = sizeof...(Types);
    };
 
    template<typename TList>
    inline constexpr size_t TypelistSizeV = TypelistSize<TList>::value;
 
    // Typelist: indexed access
 
    template<typename TList, size_t Index>
    struct TypeAt;
 
    template<typename Head, typename... Tails>
    struct TypeAt<Typelist<Head, Tails...>, 0> {
        using type = Head;
    };
 
    template<typename Head, typename... Tails, size_t Index>
    struct TypeAt<Typelist<Head, Tails...>, Index> {
        static_assert(Index < sizeof...(Tails)+1, "index out of range");
        using type = typename TypeAt<Typelist<Tails...>, Index-1>::type;
    };
 
    template<typename TList, size_t Index>
    using TypeAtT = typename TypeAt<TList, Index>::type;
 
    // Typelist-to-std::variant conversion
    namespace detail
    {
        template<typename... Types>
        auto to_variant(Typelist<Types...>) -> std::variant<Types...>;
    }
 
    template<typename TTypelist>
    using VariantOfTypelistElements = decltype(detail::to_variant(std::declval<TTypelist>()));
}