How to configure QtCreator to use Clang 19 instead of GCC

Pete Carter

I have been trying to use the modules feature is C++ but somehow there is a problem in GCC (12.2.0)
Since i could not update GCC, i decided to switch to Clang

I downloaded Clang v19:

sudo apt install clang-19 clangd-19 clang-format-19 clang-tidy-19 clang-tools-19

In QtCreator, Preferences -> Kits
I cloned the Desktop GCC kit, then modified the following:

Compiler: /bin/clang-19
Debugger: /usr/bin/lldb-19
CMake Tool: /usr/bin/cmake

everything else is the same as GCC kit

In Preferences -> C++ -> Clangd

Path to executable: /usr/bin/clangd-19

In Preferences -> Analyzer -> Clang Tools

Clang-Tidy: /usr/bin/clang-tidy-19

Now everything should be configured correctly, that's what i thought ...

I have a problems i noticed so far, I cannot find any files i include, like iostream or cassert

Pete Carter

Tried adding the libraries in the include path in CMakeLists.txt

cmake_minimum_required(VERSION 3.25)

project(learn_cpp_33 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_compile_options(-fmodules-ts)

add_executable(learn_cpp_33 src/main.cpp src/lib/helpers.cpp src/lib/data.cpp)

target_include_directories(
  learn_cpp_33 SYSTEM PRIVATE /usr/lib/llvm-19/include
                              /usr/lib/llvm-19/include/c++/v1)

include(GNUInstallDirs)

install(
  TARGETS learn_cpp_33
  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
  RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})

But now i am getting a different error:

main.cpp:1:10: In included file: 'features.h' file not found
platform.h:35:12: error occurred here

I also don't want to keep editing the cmake file like that for every project, is there a way to add this include path system wide so that qtcreator can find it automatically ?

jsulm

@Pete-Carter said in How to configure QtCreator to use Clang 19 instead of GCC:

I have a problems i noticed so far, I cannot find any files i include, like iostream or cassert

Does the project compile?
These errors you show could come from the code model.

Pete Carter

@jsulm

When i restarted the IDE, and changed the kit to clang, it now compiles.
it compiles well without trying to use modules.

However when i try to add modules support by adding add_compile_options(-fmodules-ts) in CMakeLists.txt

cmake_minimum_required(VERSION 3.16)

project(learn_cpp_7 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

add_compile_options(-fmodules-ts)

add_executable(learn_cpp_7 src/main.cpp src/lib/helpers.hpp src/lib/helpers.cpp
                           src/lib/data.hpp src/lib/data.cpp src/module_0.cpp)

include(GNUInstallDirs)
install(
  TARGETS learn_cpp_7
  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
  RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})

I get this error:

main.cpp:4:10: 'bitset' file not found

The toolchain supposed to be in /usr/lib/llvm-19/

cristian-adam

The issue is tracked at https://bugreports.qt.io/browse/QTCREATORBUG-28465

clangd doesn't currently support C++20's modules. See also https://github.com/clangd/clangd/issues/1293

It looks like progress is being made though.

Pete Carter

@cristian-adam

Thanks man

SimonSchroeder

It is not sufficient to enable modules for the compiler. Your build system (CMake in this case) needs to now about modules as well to compute the right dependencies. You have to tell CMake which files are defining modules (maybe have a look here: https://cryos.net/2024/01/c-modules-and-cmake/). Otherwise your build might fail (it also might just work right most of the time if the dependencies are accidentally compiled in the right order). If I'm not mistaken modules with CMake so far only work with the ninja generator.

The reason why the build system needs to know about that is because if the compiler is looking for a module and it hasn't been compiled yet, there is no way for the compiler to tell the build system to just build the necessary module. So, the build system has to make sure that all modules have been built (in the right order) before regular compilation starts. (It would be certainly nice if the compiler could just compile the required module as well and then continue. But this is not how it works.)