On Sat, 7 Jun 2025, 22:20 Avi Kivity via Std-Proposals, <std-proposals@lists.isocpp.org> wrote:

On Sat, 2025-06-07 at 21:57 +0300, Ville Voutilainen via Std-Proposals wrote:

On Sat, 7 Jun 2025 at 21:44, Robin Savonen Söderholm via Std-Proposals

<std-proposals@lists.isocpp.org> wrote:

You can easily implement your own version of optional that does exactly that (it would even be a simpler class to implement, although not to use). The question then is why? If you somehow think that you would gain performance out of it, then my suggestion is: implement this type of optional and show both a benchmark where it makes a significant difference and tell us the use case where this performance gain can be leveraged.

My 2 c.

That's easy. The hard part is figuring out how to deploy such a type

with the same name as its previous version that didn't use

the dummy value protocol. It's a behavioral ABI break without any

layout ABI break.

This was solved already when std::string changed its ABI. The compilers shipped the standard library compiled in both modes for a while, then dropped the old mode.

We didn't drop it, both modes are still supported (by a single binary) and in use today, and it took a year of my life and I don't want to do it again :-)

And it still has costs today for every change to std::string (as we have to decide whether to implement the new stuff for the old mode too).