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Emacs can be configured with lsp-mode and clangd to work well.

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clangd

The official clangd extension can be used for C++ comprehension.
Run cmake (Meta/ladybird.py run ladybird or similar) at least once for clangd to work, as doing so will generate the Build/release/compile_commands.json that is needed by clangd.

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lsp-mode

Configure lsp-mode with the following settings: 
(use-package lsp-mode
  :hook ((c++-mode) . lsp-deferred)
  :commands lsp
  :config
  (setq lsp-clients-clangd-args '("-j=4" "-background-index" "--log=error" "--clang-tidy" "--enable-config"))
  ;; Optionally, set the location of clangd -- See notes below for options.
  (setq lsp-clangd-binary-path "/usr/bin/clangd"))

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clangd installation options

There are a few different ways to specify which clangd to use:
1

System clangd (default)

By default, without configuration lsp-mode will try to find and use your system clangd. This is the easiest solution, but your system clangd might be out of date.
2

Manual path specification

You can manually specify any clangd binary with lsp-clangd-binary-path, as shown in the use-package example above.
3

lsp-mode managed installation

You can have lsp-mode manage your clangd installation with emacs’ lsp-install-server. This will install a clangd binary for you.
4

Build from Ladybird repository (advanced)

You can build the LLVM toolchain, including clangd, from Ladybird’s repository. This is an advanced option that is not currently documented.

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clang-format

There are multiple packages to handle auto formatting with clang-format, within emacs. Choose what works best for your setup:

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Using lsp-mode for formatting

Alternatively, this can be done without additional packages, using lsp-mode. You can use the following .dir-locals.el file placed in the project root: 
((c++-mode
  (eval add-hook 'before-save-hook #'lsp-format-buffer nil t)))
This configuration will automatically format your code on save using the Ladybird project’s .clang-format style file.

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