For C++ projects I use a language server to jump around within Emacs. For a Linux kernel which is my day job, I just use git grep, since I couldn't make the language server work across multiple architectures at the same time and plain old C is simple enough for that. I have never used auto completion, so perhaps I don't know what I am missing. At least to me, coming up with an idea how to do stuff is more time-consuming than writing it down in an editor.