Import C++ Standard Library Headers on CMake 3.30

Code
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1

Hi,

I’m trying to write a CMakeLists to compile C++ code with import <...>;, like:

// main.cpp
import <iostream>;
int main() {
  std::cout << "Hello World" << std::endl;
  return 0;
}

It can be compiled by a simple command with clang18:

/usr/local/Cellar/llvm/18.1.8/bin/clang++ -std=c++20 -fmodules main.cpp

But when I tried to write CMakeLists, like this:

# File No.1
cmake_minimum_required(VERSION 3.30)
project(Cpp20Sample CXX)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_SCAN_FOR_MODULES ON)
add_executable(main main.cpp)

# File No.2
cmake_minimum_required(VERSION 3.30)
project(Cpp20Sample CXX)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
add_executable(main main.cpp)

# File No.3
cmake_minimum_required(VERSION 3.30)
project(Cpp20Sample CXX)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_SCAN_FOR_MODULES ON)
add_executable(main)
target_sources(main PUBLIC FILE_SET CXX_MODULES FILES main.cpp)

All of them failed with a message:

error: header file <iostream> (aka '/usr/local/Cellar/llvm/18.1.8/bin/../include/c++/v1/iostream') cannot be imported because it is not known to be a header unit
2

When I searching for solutions, I found this:

Seems it’s still unsolved on CMake 3.30?

3

Yes, header units are pretty much the last thing we can support; all kinds of other patterns show up in header unit support only “more complex”.

4

It can’t, unless the distro ships precompiled header units correctly. Many distros still don’t, e.g. ArchLinux doesn’t. Here’s an example (hello.c++):

import <iostream>;
int main() { std::cout << "hello world\n"; }

$ clang++ -std=c++23 -fmodules hello.c++ -o hello
hello.c++:1:8: error: header file <iostream> (aka '/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../include/c++/14.2.1/iostream') cannot be imported because it is not known to be a header unit
    1 | import <iostream>;
      |        ^
hello.c++:2:14: error: use of undeclared identifier 'std'
    2 | int main() { std::cout << "hello world\n"; }
      |              ^
2 errors generated.

Not good. However, if we first precompile the header unit manually, as documented here, then it will work just fine:

$ clang++ -std=c++23 -xc++-system-header --precompile iostream -o iostream.pcm
$ clang++ -std=c++23 -fmodule-file=iostream.pcm hello.c++ -o hello
$ ./hello
hello world

The problem is that (1) many of these command-line flags may be clang++-specific and (2) (re)building of standard C++ header units again for each project is not a great approach. Distros should make sure that .pcm files are available out-of-the-box.