Config default C/C++ standard in VSCode with clangd and FQ bugs

Question

Since I have struggled these warnings/errors for so long. I decided to ask this question and answer myself for late comers.

Extension: vscode-clangd

1. 'auto' type specifier is a C++11 extensionclang(-Wc++11-extensions) or any "grammar belong to C++11 or later" error. How to set default C/C++ standard in vscode-clangd?

2. Invalid argument '-std=c99' not allowed with 'Objective-C++' or any header extension .h interpreted as C/C++ conflicting with your standard. How to deal with this annoying bugs?

Answer 1

Solutions

1. Three ways

   • For small projects: write a compile_flags.txt:

   -std=c++11 
   

   • For big projects: generate complie_commands.json by a build system like CMake, which can be enabled by adding this to your CMakeLists.txt file:

   set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
   

   • If no compilation database(compile_flags.txt/complie_commands.json) is found, you can do this in your vscode:

     • cmd/ctrl + shift + p: Preferences: Open User Settings （JSON）
     • Add these into your settings.json file:

     "clangd.fallbackFlags": [
         "-std=c++11",
         // do not add line below or it will cause conflict
         // "-std=c99",
       ],
     
     

     • cmd/ctrl + shift + p: clangd: Restart language server

2. You just can't set C & C++ standard into your clangd.fallbackFlags at the same time or it will conflict with each other. Two way to fix this:

   • But you can make your compiler treat your .h as C/C++ by -x

   > man clang
   
   -x <language>
            Treat subsequent input files as having type language.
   

   see: https://stackoverflow.com/a/19413311/17001641

   • or add a .clangd into your project root with these lines:

   If:
     PathMatch: .*\.cpp
   
   CompileFlags:
     Add: [-std=c++20]
   
   ---
   
   If:
     PathMatch: .*\.c
   
   CompileFlags:
     Add: [-std=c99]
   

   see: https://stackoverflow.com/a/76286667/17001641

Explaination

clangd first check for a compilation database in the directory containing the source file, and then walk up into parent directories until we find one. The compilation database can be:

1. a file named compile_commands.json listing commands for each file. Usually generated by a build system like CMake.

   • Tedious and error-prone to write by hand
   • See: format and example

2. a file named compile_flags.txt listing flags to be used for all files. Typically hand-authored for simple projects.

   • easy to use in small projects

3. If no compilation database is found, use clangd.fallbackFlags.