On Tue, Sep 6, 2022 at 8:02 AM Frederick Virchanza Gotham via Std-Proposals
<std-proposals_at_[hidden]> wrote:

> I program microcontrollers in my day job, so I'm weary of using
> dynamic memory allocation. I avoid it wherever possible.
>
> Currently I have an "interface class" [...]

        class IRS232 {
> [...] I have two classes that derive from IRS232, like this:
> class RS232_Chunked : public IRS232 { . . . };
> class RS232_Instant : public IRS232 { . . . };
> [...] So I can do the following:
> std::variant<std::nullopt_t,RS232_Chunked,RS232_Instant>
> g_coms( std::in_place_index<0u>, std::nullopt );
> IRS232 *g_pcoms = nullptr;
>
> void Initialise_Comms(bool const chunked = false) {
> if ( chunked ) {
> g_coms.emplace<1u>();
> g_pcoms = &std::get<1u>(g_coms);
> [...]


> In order to simply this, I think that the "std::variant" class should
> have a method called "common_base", which could be used as follows:
>

This is just std::visit. Here's the "very simple" version using the Null
Object pattern (i.e., creating a new dummy subclass of IRS232 to represent
"no connection"):
https://godbolt.org/z/rEW6aq744
And here's the "less simple" version:
https://godbolt.org/z/h1rjzxGKG

There are other ways to do it too, but std::visit seems like the simplest —
and it's already present in C++17.

HTH,
Arthur