Symbian OS is written in C++, excluding some internals of the kernel written in C/ARM assembly. I was told when I worked there (3-4 years ago) that the entire code base (including test code, variant code etc.) is on the order of 50 million lines of code. So definitely large. Stable? A matter of opinion.
Edit: Thanks, I didn't know there was a C++ OS out there. Though to be fair to C++ it hasn't been around as long nor been as stable as C (from C with classes to C with classes exceptions and templates.)