C++20 modules: manually specifying module providers

Usage
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1

Hi,

is there a way to manually specify what modules are provided by given library?

I.e. I precompile header modules in add_custom_command via:
“g++ -fmodule-header -x c++system-header cassert”
Then wrap it into a static library “cxx_std”

How do I tell cmake that my “cxx_std” library provides definitions of cassert and other header modules?

Is it even possible without modifications of cmake implementation?

Regards

2

Header units are not supported at all (CMake doesn’t know how to deal with a reported import <cassert>; report from dependency scanners).

3

Yes, I am aware cmake does not support header units. But I am trying to implement by hand.
Also for now I am interested in “include translation” feature of header units, i.e.
“#include ” is automatically translated into “import cassert;”

At least for gcc this is very simple, just add new entry into modmap file:
/usr/include/c++/14/cassert $<BUILD_DIR>/usr/include/c++/14/cassert.gcm

The process is as follows:

  • compiler performs usual include path resolution
  • upon success it queries module mapper whether a module named with absolute path of header file exists
  • if exists, replace the include with an import.

The problem is that I cannot seems to find a proper way of injecting such manually specified entries into module-mapper of given cmake target, and make them propagate into cmake target users.

4

CMake’s module mapper generation is completely internal to CMake. It would require coordination with the collator which is not public API. There’s no way to supplement it either.

5

I see, I guess I will have to wait for official “header units” support.

Thanks for all your modules related work in cmake and gcc!