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I'm trying to use a feature in c++17(const lambdas) without having clangd error me. I've searched online and every answer tells me to recompile clangd with a flag. Is there truly no other way?

Edit: Clangd is not the compiler. It's a language server, which is a program made to be used with IDEs that basically checks your code for errors and warnings before compiling it. See https://clangd.llvm.org .

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    clangd.llvm.org/config#compileflags Commented Sep 17, 2022 at 21:08
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    @n.1.8e9-where's-my-sharem. Read my question again, I believe it clearly states that I was asking for a solution that didn't require me to recompile clangd. Commented Sep 17, 2022 at 21:17
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    @AaronLiu did you read his link? It’s the correct answer to your question. Clangd accepts compiler flags to determine how to interpret the files. Commented Sep 17, 2022 at 23:07
  • See this answer to a similar question. No recompiling clangd required, just creating a config file in the root directory of your project. Commented Sep 18, 2022 at 4:22
  • I read it but misinterpreted it. Sorry. Commented Sep 18, 2022 at 15:28

2 Answers 2

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Answer inspired by n. 1.8e9-where's-my-share m.

From https://clangd.llvm.org/config#compileflags:

You can create a config file for clangd. In the config file, you can specify the compiler options mimicked. For my question, do this:

CompileFlags:
  Add: [-std=c++20]

Put the configuration file as .clangd in a shared parent directory of the files you want this to apply to. If you want it to apply for the entire user, make it a config.yaml file in an OS-specific directory:

  • Windows: %LocalAppData%\clangd\config.yaml, typically C:\Users\Bob\AppData\Local\clangd\config.yaml.
  • macOS: ~/Library/Preferences/clangd/config.yaml
  • Linux and others: $XDG_CONFIG_HOME/clangd/config.yaml, typically ~/.config/clangd/config.yaml.
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5 Comments

The config file is named ~/.clangd. (ref: Configuration Files)
This worked for me using the clangd Visual Studio Code extension as it just uses the machine's installation of clangd under the hood.
Unfortunately, this will also force c files to be treated as c++20 files, which is probably not desirable.
Is it possible to set the C++ version per project?
@FelixDombek @ynn’s path only works if your project file is under any ~/ or any subdirectory of it, as they’ve provided the way to specify it “per-project” for all subdirectories of ~/. I’ve edited the answer.
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As of clangd12, clangd can use a compilation database (compile_commands.json) just like clang-tidy. You can avoid hard-coding compilation flags (including language standard) if your build tool supports compilation databases.

A compilation database is an autogenerated file that contains a precise dump of compiler settings. They are used to communicate between various C++ tools.

Example with CMake 3.5+ and default clangd config

Use project source directory or a build/ subdirectory as the binary directory and enable the CMAKE_EXPORT_COMPILE_COMMANDS flag:

cmake -B build -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
# or cmake -B . -DCMAKE_EXPORT_COMPILE_COMMANDS=ON

clangd will now use the same settings as the compiler would.

Search paths

By default, when processing any C/C++ file, clangd searches for a compile_commands.json in the file's directory and all parent directories, and in the build/ subdirectory in each of those directories, as pointed out in this answer.

This behavior can be changed via clangd configuration.

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3 Comments

Thanks. I get what you mean, but could you add an example compilation database that compiles C++ files with the -std=c++20 flag?
I could add an example to the answer itself, but compilation databases are extremely machine-, project- and configuration-specific. It shouldn't really matter because they are autogenerated by build tools. If you don't use tools that could autogenerate one for you, then use your previous answer. Compilation databases are a standardized way to communicate between automatic tools, maintaining them by hand is nearly impossible. For example, they contain absolute paths.
Note that you are most likely already using a tool that can export compilation databases, at least via extensions: CMake, Bazel, most IDEs, etc.

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