So far as I can tell, it's not quite the same thing since these still have pointer semantics (and thus have to deal with aliasing etc). The in/out approach is more generic, since "in" can map to a pointer where it makes sense, and to a copy where it does not.

Better yet when you prohibit such arguments from aliasing (or at least make no-alias the default) - now the compiler can also implement "in out" by copying the value back and forth, if it's faster than indirection.

Ehh that's not quite what those are. The types being added for C++23 are designed for FFI.

Herb made a proposal for proper in/out parameters for C++ in 2020 https://youtu.be/6lurOCdaj0Y

But not “out” params, sadly.

It can return multiple values, so this doesn’t matter much for value types, but it would be nice to be able to specify that a pointer arg is an out-param sometimes and enforce that it is not read from while handling allocation in the caller.