Setting up Emacs for Qt (C++) development

3 minute read

Ok starting with vanilla Emacs out of the box on Ubuntu, what does it take to setup to have a decent development experience?

To start let’s create a minimal Qt Project.

main.cpp:

#include <QCoreApplication>
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    return a.exec();
}

Let’s just use QMake for compilation, so this would just be:

$ qmake -project
$ qmake
$ make
g++ -c -pipe -O2 -Wall -W -D_REENTRANT -fPIC -DQT_DEPRECATED_WARNINGS -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -isystem /usr/include/x86_64-linux-gnu/qt5 -isystem /usr/include/x86_64-linux-gnu/qt5/QtGui -isystem /usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o main.o main.cpp
g++ -Wl,-O1 -o hello main.o   /usr/lib/x86_64-linux-gnu/libQt5Gui.so /usr/lib/x86_64-linux-gnu/libQt5Core.so /usr/lib/x86_64-linux-gnu/libGL.so -lpthread   
$ ./hello 

So what does this look like in emacs?

Initial State

Huh, I was expecting Emacs to be white by default. Emacs picks up something default from the underlying theme?

For some, this would be enough. Heck it was enough for me for years. You can compile, debug with GDB, set breakpoints (C-x c-a c-b) etc. What more do you want?

Yeah right, that fancy stuff like completion.

LSP

Enter lsp-mode. It is a client for LSP, Language Server Protocol. What does that mean? It means that there is a server running somewhere that looks at your code, compiles and analyzes it, translates this data into a common format and provides it to clients such as Emacs or VSCode to display ontop of the source code file.

To make things a bit more confusing lsp-mode in Emacs sometimes provides the server part which makes whole operation is completely transparent. Sometimes it will prompt you to install some external language server tool so that it can start it when necessary. Sometimes there’s a lot of configuration to be done before it consents to doing anything.

But let’s start with LSP. Let’s install it MELPA (we need to add it as a source first). Let’s also use use-package so that things won’t immediately turn into a mess:

MELPA and use-package. Let’s also lsp-mode along with company to show us errors and completions etc.

(require 'package)
(setq package-enable-startup nil)
(add-to-list 'package-archives
             '("melpa" . "https://melpa.org/packages/"))
(package-initialize)
(unless (package-installed-p 'use-package)
  (package-refresh-contents)
  (package-install 'use-package))

(use-package company
  :ensure t
  :config
  (setq company-show-numbers t)
  (setq company-tooltip-align-annotations t)
  (setq company-tooltip-flip-when-above t)
  (global-company-mode))

(use-package company-quickhelp
  :ensure t
  :init
  (company-quickhelp-mode 1)
  (use-package pos-tip
    :ensure t))

(use-package lsp-mode :ensure t)
(use-package lsp
  :ensure lsp-mode
  :config
  (require 'lsp-clients)
  (add-hook 'lsp-after-open-hook 'lsp-enable-imenu)
  :init
  (setf lsp-eldoc-render-all nil)
  (setq lsp-inhibit-message t)
  (setq lsp-message-project-root-warning t))

(use-package lsp-ui :ensure t :commands lsp-ui-mode)
(use-package ccls
    :ensure t
    :hook ((c-mode c++-mode objc-mode cuda-mode) .
         (lambda () (require 'ccls) (lsp))))

 (use-package company-lsp :ensure t)

If you run reload the main.cpp file or run lsp while in it you’ll be greeted with this:

no server

The provided link will land you to the home page of lsp-mode which is less than helpful. But it does say that either ccls or clangd server will support a cpp file. It leaves up to you to figure out which you want and that they are external applications.

Let’s go with ccls, so we well need to apt install ccls

After this if we reopen (or run lsp our main.cpp we will be greeted with errors: after lsp

Almost! The final thing missing is to have a Compile Commands json file for ccls, otherwise none of the includes work.

For this, we can use Bear, so apt install bear

Then on the directory where we have our project:

$ make clean # (make sure it compiles)
$ bear make

This creates the required compile_commands.json. Reload ..and..the error doesn’t budge. Hmm.

Time to debug. Let’s look at what *ccls::stderr*buffer says.

First line:

19:42:31 ccls           initialize.cc:262 I initialize in directory /home/lahtela/tmp with uri file:///home/lahtela/tmp

That doesn’t look right, my sample is in /home/lahtela/tmp/hello. There’s some file in tmp that makes ccls detect it as the project root directoy. Maybe .ccls-cache, none of the files listen in the FAQ are there anyway. Let’s move our sample somewhere else.

That’s better, no errors!

Working lsp

Success! Now we have all that completion goodness. For me this is pretty much enough. You can get an even bigger IDE like with things like treemacs.

As a general observation it’s the server implementation that contains the more detailed usage instructions, not lsp-mode. For C++ for example look at the ccls lsp-mode instructions.

Tagged under: emacs Qt C++

Twitter