On Wed, Aug 23, 2023 at 7:47 AM Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]>
wrote:

> On Wed, Aug 23, 2023 at 7:39 AM Frederick Virchanza Gotham via
> Std-Proposals <std-proposals_at_[hidden]> wrote:
>
>> On Wed, Aug 23, 2023 at 4:10 PM Ville Voutilainen <
>> ville.voutilainen_at_[hidden]> wrote:
>> >
>> > I wonder what happened to the third strategy, which is to pass as the
>> > argument of existing emplace an object
>> > that will perform the function invocation in its conversion operator
>> > to the optional's element type, which then
>> > allows doing this without any library or language changes, and has
>> > field experience as a solution to this problem.
>>
>>
>> Do you mean something like the following?
>> GodBolt: https://godbolt.org/z/PfM1EKaWo
>>
>
> Yes, but it can be done much simpler.
> https://godbolt.org/z/Gr9sT6Ga9
>
> #include <optional>
> #include <mutex>
>
> template<class F>
> struct SCSE {
> F f_;
> operator auto() const { return f_(); }
> };
>
> int main(void) {
> std::optional<std::mutex> om;
> om.emplace(SCSE([] {
> return std::mutex();
> }));
> }
>
> The deficiency with the current state of the world is that even SCSE
> doesn't help at all with returning immobile objects; you can't use SCSE to
> return a locked mutex, for example.
>

...and of course you don't need the SCSE at all, if you have a constructor
that already does what you want.

https://godbolt.org/z/fWz3EzWqx

int main(void) {
std::optional<std::mutex> om;
om.emplace();
}

The reason to use the SCSE is if you have a constructor that's non-public
(so `optional::emplace` can't see it), or if you have a
factory-function-that's-not-a-constructor (so `optional::emplace` isn't set
up to use it).

–Arthur

>