Eglot based Emacs C++ IDE with clangd

2019-01-07

I have an old post documenting my first attempt at turning Emacs into a C++ IDE with clangd. That post describes using two packages: lsp-mode and lsp-clangd. Those packages have evolved and now clangd usage is built into lsp-mode, so the post is a bit outdated. I've also started to use Eglot (see previous post for my Eglot Python IDE). So, let's put together an updated setup:

Requirements

First, one needs to have clangd installed. These days, the 8.0 release of LLVM is a few months away, but clangd (part of the clang-tools-extra LLVM project) is in rapid development and the master of the repository should be used. The installation instructions from the LLVM documentation are easy to follow.

My C++ development happens on multiple machines. In my Emacs configuration I keep a simple variable around to point to wherever clangd is installed on various machines.

(defvar ddavis-clangd-exe (executable-find "clangd")
  "clangd executable path")

By default I'm letting Emacs find it, but I have things like this sprinkled around my configuration (pointing to a specific LLVM installation not in my PATH):

(when (string= (system-name) "pion")
  (setq ddavis-clangd-exe "~/software/llvm/releases/master/bin/clangd"))

Eglot setup

Eglot uses project.el, but I use Projectile, so I start by defining a function that will tell project.el to find a project via Projectile, thanks @wyuenho on GitHub:

(defun dd/projectile-proj-find-function (dir)
  (let ((root (projectile-project-root dir)))
    (and root (cons 'transient root))))

Now I have a function I call when I'm ready to start digging into a C++ project which has an associated compile_commands.json:

(use-package eglot
  :ensure t)

(defun dd/cpp-eglot-enable ()
  "enable variables and hooks for eglot cpp IDE"
  (interactive)
  (setq company-backends
        (cons 'company-capf
              (remove 'company-capf company-backends)))
  (with-eval-after-load 'project
    (add-to-list 'project-find-functions
                 'dd/projectile-proj-find-function))
  (add-to-list 'eglot-server-programs
               `((c++-mode) ,ddavis-clangd-exe))
  (add-hook 'c++-mode-hook 'eglot-ensure))

Ensure that Eglot is installed via use-package.
Ensure that the completion-at-point backend is used by company (bring it to the front of the company-backends list).
Ensure that project.el uses Projectile to find my project definition (this is because I usually have C++ projects using git submodules).
Add my clangd executable to the eglot-server-programs list.
Add the hook to automatically start Eglot.

If I don't want the hook anymore, I use this very simple function:

(defun dd/cpp-eglot-disable ()
  "disable hook for eglot"
  (interactive)
  (remove-hook 'c++-mode-hook 'eglot-ensure))