TL;DR: Flux, my C++20 sequence-orientated programming library, now has experimental support for being used as a module. Provided you’re using a recent compiler and don’t mind a few rough edges, you can say import flux and get going.

The future is (almost) here!

Trying it out

If you fancy giving it a try, you’ll need a recent version of one of the three major compilers: I’ve got it working with Clang 16, GCC 13.1 and MSVC 17.6, but slightly older compilers might work too.

First, grab a fresh checkout of Flux from Github. Then cd to the checkout directory and save the following as modules_test.cpp:

import flux;

int main()
    constexpr int arr[] = {1, 2, 3, 4, 5};
    static_assert(flux::sum(arr) == 15);

Clang 16

Starting in the top-level directory of the Flux checkout, the first step is to compile the binary module interface (BMI) with:

> clang++ -std=c++20 -I include/ --precompile -x c++-module module/flux.cpp

(You could also provide -DNDEBUG here to compile the library without extra debug checks, or your preferred Flux configuration flags if you wish.)

This should generate a file called flux.pcm. Next, we need to generate an object file to link to:

> clang++ -c flux.pcm

which should output flux.o. Finally, we can compile our test program:

> clang++ -std=c++20 -fprebuilt-module-path=. flux.o modules_test.cpp -o modules_test

If this compiles and outputs the modules-test executable, then congratulations! Everything is working.

GCC 13

For GCC, we need to provide the -fmodules-ts flag to enable modules support: just selecting C++20 mode is not enough.

> g++-13 -std=c++20 -fmodules-ts -I include/ module/flux.cpp -c

This should generate both the BMI (in a folder called gcm.cache/flux.gcm) and the flux.o object file.

Next we can compile the test file with:

> g++-13 -std=c++20 -fmodules-ts flux.o modules_test.cpp -o modules_test

Note that needs to be done from the directory that contains the gcm.cache folder. Presumably it’s possible to persuade GCC to look elsewhere for BMIs, but I wasn’t able to find the right options.

MSVC 17.6

For MSVC, open up a Visual Studio developer prompt and cd to your Flux checkout. Then run:

> cl.exe /std:c++20 /EHsc /I include /c /interface /TP module\flux.cpp

This will generate the flux.ifc BMI and flux.obj object file.

We can then compile the test program with

> cl.exe /std:c++20 /EHsc flux.obj module_test.cpp

Note that at the time of writing, MSVC doesn’t seem to like the flux::ref() function when using Flux via an import. It works fine via #include, and when importing in Clang and GCC, so it might just be a compiler bug.

As a workaround, you can say flux::from(std::cref(seq)) instead of flux::ref(seq), which is semantically equivalent (though quite ugly).


If you’re using Clang or MSVC and are feeling particularly brave, you can also try out modular Flux using CMake.

You’ll need to add Flux as a subproject and configure it with FLUX_BUILD_MODULE=On, which will add an extra library target called flux-mod. You should then be able be able to use target_link_libraries() to link your own executable with flux-mod, which will take care of adding all the necessary build commands.

Currently this uses Victor Zverovich’s modules.cmake rather than CMake’s very new built-in module support, although that might change at some point as the built-in CMake support matures.

Modularising the code

Flux is a C++20 library that uses lots of cutting-edge features, but the lack of compiler and build system support meant that I had to write it as a “traditional” single-header library rather than using modules.

Then a few days ago I was watching Daniel Ruoso’s C++Now 2023 talk, “The Challenges of Implementing C++ Header Units”. The whole talk is great (if a bit depressing), but I was particularly interested in a slide near the end, where Daniel presents a simple way for authors of existing (non-modular) libraries to produce a wrapper module:

Screenshot from CppNow

That… actually seemed pretty easy, so I thought I’d give it a go. I whipped up very quick test file:


#include <flux.hpp>

export module flux;

export namespace flux {
    // Just export a few names to see if this works
    using flux::sequence;
    using flux::filter;
    using flux::map;
    using flux::write_to;

Amazingly, once I’d figured out the correct Clang command-line incantations, it seemed to work – which was very different to my experiences of trying modules up to this point. Flushed with success, I went through and added all of the other public names to the export namespace block: about 300 in all.

I now had a modular version of Flux that worked with Clang and MSVC. GCC didn’t like it for some reason – and to be honest, I didn’t like it all that much either. Duplicating all the public names in two places seemed very inelegant, and it would be annoying to have to remember to update the module file for every public symbol added to the library.

Hunting for an alternative, I looked at the only major project I know of that currently provides a public module: Victor Zverovich’s {fmt}. Following this example, I went through an added a FLUX_EXPORT macro to all the public names in the library, which expands to export if FLUX_MODULE_INTERFACE is defined, and nothing otherwise.

I then re-wrote the module file as


#include <array>
#include <compare>
/* ... all our other stdlib includes ... */

export module flux;

#include <flux.hpp>

If I understand correctly – and that’s quite a big “if”, as I’m still new to modules! – this attaches all of the names in the library with the flux module, which seems like… what we want? But by first #include-ing all of the standard library headers outside of the module purview, we avoid taking ownership of any of the stdlib stuff.

I tried out the new approach and found that not only did it work just as well with Clang and MSVC, but GCC was also much happier now.

…but is it fast?

The short answer: yes.

I was interested to see what, if any, speedup we’d get using Flux via a module versus traditional header. As a test, I used a small program which solves the Sushi For Two problem from Conor Hoekstra’s Top 10 Algorithms set (which Conor also talks about in his fantastic “New Algorithms in C++23” talk – look out for it when it drops online!). Here it is:

#include <algorithm>
#include <vector>

import flux;
#include <flux.hpp>

constexpr int sushi_for_two(std::vector<int> const& sushi)
    return 2 * flux::ref(sushi)

static_assert(sushi_for_two({2, 2, 2, 1, 1, 2, 2}) == 4);
static_assert(sushi_for_two({1, 2, 1, 2, 1, 2}) == 2);
static_assert(sushi_for_two({2, 2, 1, 1, 1, 2, 2, 2, 2}) == 6);

int main()

The #includes at the top are intended to replicate a somewhat realistic scenario – most real-world TUs are going to use some standard library facilities, after all.

I then used Clang’s -ftime-trace facility to look at the internal timings during compilation:

  • Without modules, the #include <flux.hpp> line took around 570ms to process on my laptop (of which around half was #include-ing other standard library headers, and the rest parsing the Flux code itself)

  • With modules, the import flux step took 57ms – almost exactly ten times faster!

Of course, this is only a single, unscientific test with one compiler on one machine, but the numbers certainly look encouraging.

Looking forward

All this playing around has finally – three years after C++20 was finished! – got me excited about modules. There’s still a very long way to go yet in terms of seamless compiler and build system support, but things seem to be heading in the right direction.

Flux will remain a header-based library for the foreseeable future, but I’ll keep trying to improve the modules story. And who knows, maybe by the time C++26 rolls around we’ll finally be able to (mostly) consign #include to the history books.