[quotemstr]

The C++ standard library is a bit different from a platform API. The C++ standard library is a template system designed to conform exactly to your particular program. It should always be statically linked. I see zero advantage to dynamic linking of libc++. A platform API, on the other hand, is designed to stable, safe, and flexible, not fast --- because programs in general shouldn't be calling into the system so much. The performance costs we accept in a platform API (e.g., user to kernel privilege transition) would be totally unacceptable as part of an STL data structures implementation.

[pjmlp]

Except when OSes happen to be written in C++, like most modern non-UNIX/POSIX clones, and security updates come into play.

[quotemstr]

The OS implementation language is irrelevant: there's no reason that applications and the OS core need to use the same C++ standard library.