C++ Observer Design Pattern Vs IPC Semaphore

I theoretically understand the observer design pattern. The purpose, in short, is for notifying the change of a data state.

Can't we achieve the same result using in-build native provisions for signalling like IPC synchronisation mechanism, say using semaphore?. And if you want to give a platform-independent interface also, no issues, we can provide common interface base class and underlying implementation can be done by providing dedicated sub-class for each OS.

Thanks in advance for the reply.