They are very different languages with different objectives. Both of them started with the goal to provide OO to the C language because It was used for everything at that time.

C++ was designed with performance in mind. The main goal was to be as efficient as C and It means to not use any runtime.

Objective-C was designed to be source compatible with C and extends the language with a Smalltalk like syntax for OO. It uses a runtime which provides things like reflexion and GC.