> Additionally, in C++ strings of less than 23 (IIRC) bytes are not heap allocated (they reuse fields in the existing structure).
Not quite. SSO is a lib-specific optimisation. LLVM's libc++ has a 22 characters SSO[0], recent MSVC and recent GNU libstdc++ have 15 characters SSO, Apache and older libstdc++ didn't have SSO at all (and used COW).
The SSO-23 POC/proposal does provide a 23-characters SSO for a 24 bytes string structure (such as LLVM's, MSVC 2013 and modern G++ strings are 32 bytes and SSO-23's scheme provides space for 31 characters in the same stack space).
[0] all capacities exclude the final NUL as it's required and unusable outside of C compatibility
Not quite. SSO is a lib-specific optimisation. LLVM's libc++ has a 22 characters SSO[0], recent MSVC and recent GNU libstdc++ have 15 characters SSO, Apache and older libstdc++ didn't have SSO at all (and used COW).
The SSO-23 POC/proposal does provide a 23-characters SSO for a 24 bytes string structure (such as LLVM's, MSVC 2013 and modern G++ strings are 32 bytes and SSO-23's scheme provides space for 31 characters in the same stack space).
[0] all capacities exclude the final NUL as it's required and unusable outside of C compatibility