Clangd based Emacs C++ IDE


Tags: cpp emacs

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I’ve seen a lot of posts on the Emacs and C++ subreddits over the last few months related to Emacs as a C/C++ IDE. If you give the topic a quick googling you’ll see a lot of tutorials that walk through using cquery, lsp-mode, rtags, ggtags, irony, company, ycmd, etc. (obviously there are a number of options out there and multiple blog posts and tutorials for each). I’ve personally tried using cquery and rtags (both libclang based) in combination with company-mode. Playing with those packages produced a hacked up Emacs init file and I didn’t really know what I was doing at the time. I was never comfortable with the black box I created for myself – so I decided to clean it up and start over after some research.

I’ve recently landed on a new setup using a combination of lsp-mode, company, and lsp-clangd. As is clear from the package name and post title, this method takes advantage of the LLVM/Clang tool clangd (which is very much in development).

Here’s a quick rundown of the new configuration:

Ensure that company-lsp is installed and enable company-mode (I choose a global configuration):

(use-package company-lsp
  :ensure t
  (require 'company-lsp)
  (push 'company-lsp company-backends)
  (add-hook 'after-init-hook 'global-company-mode))

Ensure that lsp-mode and lsp-ui are installed and required:

(use-package lsp-mode
  :ensure t
  (require 'lsp-mode))

(use-package lsp-ui
  :ensure t
  (require 'lsp-ui))

Unfortunately lsp-clangd isn’t in melpa yet, so I cloned it to my .emacs.d directory and make sure to point to it (while writing this post there is an open GitHub PR to add lsp-clangd to melpa). Be sure to set the proper clangd executable path and add a hook to C++ mode to enable it:

(use-package lsp-clangd
  ;; for macOS
  (when (equal system-type 'darwin)
    (setq lsp-clangd-executable "/usr/local/opt/llvm/bin/clangd"))
  ;; for Fedora box
  (when (string= (system-name) "proton")
    (setq lsp-clangd-executable "/home/ddavis/Software/llvm/head/bin/clangd"))

  (add-hook 'c++-mode-hook #'lsp-clangd-c++-enable))

Like I said, Clangd is under heavy development, so expect some imperfections. For example, using the version shipped with the LLVM 6.0.0 release wasn’t working with header files. I went ahead and built a bleeding edge installation (using brew install --HEAD llvm on macOS and building from the trunk of their svn repositories on a Fedora machine; read how to do that here) and that fixed the problem.

I use this setup in combination with compile_commands.json files which are produced by CMake. This file must be kept at the project root (using Projectile with a .projectile file at the project root helps when using git repositories with submodules; lsp-mode appears to handle that nicely).

I’m still by no means an expert, but it was a good learning experience and I no longer have a black box from copying and pasting from other’s Emacs init files. I have code completion and inter/intra-project file and definition jumping – the two big features I like to add to my C++ development setup in Emacs.