The implementation is a reasonable start but unaligned<trivial> needs to
itself be trivial (to allow composition of structs containing unaligned
members inside other unaligned<> ), so it gets just a tad more interesting.

On Thu, Dec 7, 2023, 08:01 Lénárd Szolnoki via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On Wed, 2023-12-06 at 17:01 +0000, Frederick Virchanza Gotham via Std-
> Proposals wrote:
> > On Wed, Dec 6, 2023 at 4:52 PM Gašper Ažman wrote:
> > >
> > > ++ for unaligned<trivial>
> >
> >
> > So how would "std::unaligned<Trivial>" work? Would it lack tail
> > padding? Or would it lack tail padding and also lack padding between
> > members?
>
> std::unaligned would be applied to the members, not the enclosing
> class, like so:
>
> struct Monkey {
> std::unaligned<long> l;
> char ch;
> };
> static_assert(sizeof(Monkey) == sizeof(long) + sizeof(char));
>
> std::unaligned could be implemented something like this:
>
> template <typename T, std::size_t alignment = 1>
> requires (std::popcount(alignment) == 1)
> class unaligned {
> static_assert(std::is_trivially_copyable_v<T>);
>
> using storage_t = std::array<std::byte, sizeof(T)>;
> alignas(alignment) storage_t m_storage;
>
> public:
> constexpr unaligned(T t) noexcept
> : m_storage(std::bit_cast<storage_t>(t))
> {}
>
> constexpr operator T() noexcept {
> return std::bit_cast<T>(m_storage);
> }
> };
>
> You also identified the problem of taking address of packed members and
> the potential need for special treatment of them in the language. If
> implemented like above then you don't need special treatment in the
> language, you can't take the address and get a `long *` out of an
> unaligned<long>.
>
> >
> > So if I have a trivial standard-layout struct as follows:
> >
> > struct Monkey {
> > long unsigned a;
> > char b;
> > // padding here?
> > long double c;
> > char d;
> > // padding here?
> > float e;
> > char f;
> > // padding here?
> > };
> >
> > Then what will std::unaligned<Monkey> do? Will it take out just the
> > tail padding, or will it take out the 3 pieces of padding?
>
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