nix-cpp-hello

This project is an example of how to set up a simple C++20 project with Nix and clangd. It may be useful if you're trying to develop with C++ on NixOS.

For more background, check out this NixOS Discourse thread.

VSCode C++ Development Workflow on NixOS

Follow the instructions below to set up a basic C++ project in VS Code on NixOS. You can clone this repo and work within it, or use it as a reference and write Nix expressions for your own project.

Prerequisites

This example assumes you're using the clangd language server and Nix flakes.

First set up VS Code with clangd and direnv. The clangd extension will provide syntax highlighting, autocomplete, and other language server functionality. The direnv extension will make development dependencies from your Nix devShell (like clangd from clang-tools) available within VSCode.

NixOS VSCode configuration.nix example (without home-manager):

{ config, pkgs, ... }:
{
  # ...

  environment.systemPackages = with pkgs; [
    (vscode-with-extensions.override {
      vscodeExtensions = with vscode-extensions; [
        llvm-vs-code-extensions.vscode-clangd
        mkhl.direnv
      ];
    })
  ];
}

home-manager VSCode configuration example:

{ config, pkgs, ... }:
{
  # ...

  programs.vscode = {
    enable = true;
    extensions = with pkgs.vscode-extensions; [
      llvm-vs-code-extensions.vscode-clangd
      mkhl.direnv
    ];
  };
}

Configuring clangd

clangd requires a configuration file called compile-commands.json to help understand your source code. The standard recommendations to generate this file (like the bear tool) don't work on NixOS. Thankfully, mini_compile_commands can help us automatically generate a compile-commands.json file.

The default devShell in this repo's flake provides an environment with mini-compile-commands.

To generate compile-commands.json, run the following in the project directory:

$ nix develop
$ mini_compile_commands_server.py &
$ pid=$!
$ make
$ kill -s SIGINT $pid

You can test that everything works by opening this project folder in VS Code, and verifying that there are no red squiggles or errors in the hello.cpp file.

Repeat this step whenever your dependencies or compile flags change.

Acknowledgements

Daniel Barter created the mini_compile_commands project, and provided very helpful guidance on the NixOS discourse. Thanks Daniel!

The hello.cpp example file is copied from LearnCPP's C++20 compatibility check. If you're just getting started with C++, LearnCPP is a great resource.