Support 'import std'

[igormcoelho]

The website cppinsights is very useful!
Can it support 'import std;' since GCC 15 and Clang 19 already support it?
Compilation can be done manually or with CMake 4.0... it would be helpful to incentive more people to use this recent but groundbreaking feature.
Regards!

[andreasfertig]

Hello @igormcoelho,

thanks for bringing this up!

I will give a broader answer to track the implementation state.

At the time of writing the answer (June 2025), the following issues exist:

For the C++ Insights binary:

• libc++ (Clang) declares module support as experimental (https://libcxx.llvm.org/Modules.html)

Modules are an experimental feature. It has additional build requirements and not all libc++ configurations are supported yet. The work is still in an early development state and not considered stable nor complete

• There is no GCC-15 and libstdc++-15 package available for Ubuntu 24.04 required for the CI pipeline and later the web-site.
• C++ Insights does not support ExportDecl and ImportDecl
• C++ Insights adds the flag -fexperimental-library if libc++ is used and -nostdinc++ under macOS. As modules preserve the environment, at least the first one must match the module build; otherwise we get a compile-time error.
• There is also the question of what exactly could and should C++ Insights support for modules.
  • Just support import in a .cpp file?
  • Transform a module (.cppm)?
    For the website, this would mean that users need a way to select whether they transform a module or a .cpp.

For the website:

• The std modules must be compiled and copied into the appropriate Docker container, github.com/andreasfertig/cppinsights-container/ for the website. The caveat here is that this container does intentionally not contain any compiler. The module build-step must happen outside, but with the same compiler, C++ Insights was built and the same libstdc++ and libc++ as well as the same arguments (see -fexperimental-library above). This step should be possible within the C++ Insights build itself.
• When the website executes C++ Insights, it invokes the run_in_docker.sh script from the above-mentioned repo. First tests indicated that Clang appears to be fine with -fmodule-file=std=std.pcm even in C++17 mode. However, whether this requires more resources is unclear. What I don't want is to implement a import scanner in the script or before that. I also want the user to be able to adjust this argument, so silently adding it in C++ Insights is a bad option, I think.
• Even if the above works without any overhead, when we want to switch between standard libraries, the script needs to know which library is used and supply the pre-compiled module based on that information.

Andreas