Objective-C and C++ are very different apart from their C ancestry. C++ doesn't make an especially good Obj-C replacement, and Obj-C is probably no better at the situations where C++ excels than raw C. Objective-C is a higher-level, dynamic-ish, highly-OO extension to C. (great for GUIs, not so great for writing drivers) C++ is quite low level, not dynamic at all, and not overly OO. C++ is very expressive in many situations due to its generic and automatic resource management features, which Obj-C lacks completely. (GUI code is typically horrendous, driver writing is nicer than in C)
"(great for GUIs, not so great for writing drivers)"
Probably the most succinct description of the relative strengths of Objective C and C++. Objective C for GUIs, C++ for drivers and embedded development.
NeXTStep drivers were written in Objective-C using a framework called DriverKit, and this was on 90s machines.
Objective-C is just C; there is no inherent performance penalty other than the dynamic dispatch of messages. In situations where that would actually be an issue, you'd be avoiding object-oriented features in C++ as well.
However, driver performance bottlenecks are typically in I/O throughput, not the language the driver was compiled in. The fact OS X drivers are written in a C++ subset likely has to do with the demands of device manufacturers coming over from MacOS 9, just like the existence of Carbon was to appease application developers like Adobe.
"great for GUIs, not so great for writing drivers"
NeXTStep had DriverKit, a framework for writing drivers in Objective-C, and this was on 1990s-era PCs. Driver performance is I/O-bound and typically has little to do with the language the driver was compiled in.
