TightPair.h

#pragma once
 
#include <type_traits>
 
namespace ll::data {
 
constexpr inline struct ZeroThenVariadicArgs {
} zeroThenVariadicArgs;
constexpr inline struct OneThenVariadicArgs {
} oneThenVariadicArgs;
 
template <class Base, class Other, bool = std::is_empty_v<Base> && !std::is_final_v<Base>>
class TightPair final : private Base {
    Other sec;
 
public:
    using first_type  = Base;
    using second_type = Other;
 
    template <class... Ts2>
    constexpr explicit TightPair(ZeroThenVariadicArgs, Ts2&&... val2) noexcept(
        std::is_nothrow_default_constructible_v<Base> && std::is_nothrow_constructible_v<Other, Ts2...>
    )
    : Base(),
      sec(std::forward<Ts2>(val2)...) {}
 
    template <class T1, class... Ts2>
    constexpr TightPair(OneThenVariadicArgs, T1&& val1, Ts2&&... val2) noexcept(
        std::is_nothrow_constructible_v<Base, T1> && std::is_nothrow_constructible_v<Other, Ts2...>
    )
    : Base(std::forward<T1>(val1)),
      sec(std::forward<Ts2>(val2)...) {}
 
    constexpr first_type& first() noexcept { return *this; }
 
    constexpr first_type const& first() const noexcept { return *this; }
 
    constexpr second_type& second() noexcept { return sec; }
 
    constexpr second_type const& second() const noexcept { return sec; }
};
 
template <class Base, class Other>
class TightPair<Base, Other, false> final {
    Base  fst;
    Other sec;
 
public:
    using first_type  = Base;
    using second_type = Other;
 
    template <class... Ts2>
    constexpr explicit TightPair(ZeroThenVariadicArgs, Ts2&&... val2) noexcept(
        std::is_nothrow_default_constructible_v<Base> && std::is_nothrow_constructible_v<Other, Ts2...>
    )
    : fst(),
      sec(std::forward<Ts2>(val2)...) {}
 
    template <class T1, class... Ts2>
    constexpr TightPair(OneThenVariadicArgs, T1&& val1, Ts2&&... val2) noexcept(
        std::is_nothrow_constructible_v<Base, T1> && std::is_nothrow_constructible_v<Other, Ts2...>
    )
    : fst(std::forward<T1>(val1)),
      sec(std::forward<Ts2>(val2)...) {}
 
    constexpr first_type& first() noexcept { return fst; }
 
    constexpr first_type const& first() const noexcept { return fst; }
 
    constexpr second_type& second() noexcept { return sec; }
 
    constexpr second_type const& second() const noexcept { return sec; }
};
} // namespace ll::data