Symbian OS was a microkernel. For each functionality, you would run a "server" with determined privileges, your apps would connect as clients and make use of the functionality that the server was providing them with - i.e. audio, Bluetooth, filesystem access etc.

It was very efficient, a 200 MHz Nokia could run respectable loads. But the API was hell, which wasn't caused by the microkernel architecture, but rather by inheriting mid-90s EPOC idioms.

With a normal C++ stdlib, that would have been a very fine environment indeed.