Differences in .clangd vs compile_commands.json in determining the path to the system headers

[palapapa]

With a .clangd file, you can specify the Compiler property to make clangd extract its include paths. Clangd also does the same automatically with a compile_commands.json. It seems like these two files have an overlap in their features. How do they affect each other in regards to determining the include paths?

In general, how do .clangd and compile_commands.json differ? Why wouldn't I always use .clangd to configure clangd?

[i-ky]

How do they affect each other in regards to determining the include paths?

I believe that Compiler from .clangd file can override the compiler executable specified in compile_commands.json. This may be useful if, for example, "real" compiler from compile_commands.json does not behave like GCC or clang, has implicit defines, etc. Compiler allows you to point clangd to a script that will fake the compiler output so that clangd can extract include paths, defines, etc.

In general, how do .clangd and compile_commands.json differ? Why wouldn't I always use .clangd to configure clangd?

The biggest difference is that compile_commands.json is typically generated by build system (e.g. CMake) or special tools (e.g. Bear). .clangd is typically hand-written.

I would always prefer compile_commands.json, because it keeps clangd knowledge of project's compilation options in sync with build system. But I would still have .clangd to add extra compiler warning options, enable clang-tidy checks, etc.

[HighCommander4]

I believe that Compiler from .clangd file can override the compiler executable specified in compile_commands.json. This may be useful if, for example, "real" compiler from compile_commands.json does not behave like GCC or clang, has implicit defines, etc. Compiler allows you to point clangd to a script that will fake the compiler output so that clangd can extract include paths, defines, etc.

Yep, that's right.

Another use case for Compiler is to specify the compiler (usually for the purpose of querying system include directories using --query-driver) for projects that don't have a compile_commands.json file.

In general, how do .clangd and compile_commands.json differ? Why wouldn't I always use .clangd to configure clangd?

To add to what @i-ky wrote, the capabilities that compile_commands.json has which .clangd doesn't are:

• Providing clangd with a list of source files in the project (namely, the entries in the compile_commands.json) so clangd knows what to index. This is a requirement for clangd's background indexer, which powers many cross translation unit features like finding references across the project.
• Specifying different flags for different files. By comparison, .clangd only gives you directory-level granularity. (I suppose you could get file-level granularity with PathMatch but that seems a bit tedious.)