lsp-mode or eglot and company combined with clangd works very well. I also just use the built in "c++-mode" for the major mode. None of these packages are buil

lsp-mode or eglot and company combined with clangd works very well. I also just use the built in "c++-mode" for the major mode. None of these packages are built into Emacs, but eglot and company are on elpa which means they are a "package-install" command away. melpa packages (which lsp-mode is one of) are also pretty trivial to install, it just requires adding melpa to the package archive list variable then installing like normal. Some Linux distros have these available to install with the system package manager. Even Debian has a decent bit of Emacs packages available. I personally use my distros system package manager to manage my Emacs packages since it has everything I need available.