Hello,
I’ve seen this sort of thing before as a “sentinel optional” where one
value is set aside to represent null (typically the min or max value for
integer types). It would require a customization point to provide sentinel
values for other types. I’m not sure this is a good fit for the standard
library but I’ve found it useful before. It tends to be a fairly small
optimization relative to std::optional.

Jeremy

On Sat, Jun 7, 2025 at 04:33 Avi Kivity via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Consider std::optional<T>.
>
> The move constructor has to check if the moved-from optional is
> engaged, and if so, move-construct the contained T. Similarly other
> special functions have to take actions conditionally. The destructor
> has to check if the optional is engaged and only invoke T::~T() if
> that's the case.
>
> However, if we had a way to cheaply create a T, we could use that and
> avoid all the conditionals. Let's say the protocol is
>
>
> template <typename T>
> struct construct_empty {};
>
>
> template <>
> struct construct_empty<my_lovely_type> {
> static void operator()(my_lovely_type* where) noexcept;
> };
>
> If construct_empty is specialized for a T, then optional<T>'s default
> constructor could initialize the contained T whether or not the
> optional is engaged or not, and all the conditionals for the special
> methods would disappear. For trivially constructible standard types,
> construct_empty would do nothing. For many standard types, we could
> call the default constructor. The user could opt-in for cheaply
> constructible user types, often just calling the default constructor.
>
> This seems related to trivial relocation, just from the other side as
> it were. Maybe opt-in should also be via an attribute.
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>