Re: On Boost and modules: second part

From: John Maddock (jz.maddock_at_[hidden])
Date: 2024-04-19 18:29:23

On 18/04/2024 14:48, Ruben Perez via Boost wrote:
> Hi all,
>
> Following the recent discussion on Boost and C++20 modules, I've been
> performing further investigations on the possible benefits and costs
> of offering module support in Boost. I've summarized my findings here:
> https://anarthal.github.io/cppblog/modules2

Thank you Ruben for a nice summary.

I've also been experimenting with modules in Regex - it's a smaller more 
confined effort than the one I also started in Math - but even so it's 
proved to be extremely time consuming and rather frustrating.  The 
current state of play in the draft PR here: 
https://github.com/boostorg/regex/pull/174 is as follows:

1) It more or less works with latest MSVC and clang.  Latest GCC fails 
hopelessly.

2) Build time improvements are similar to yours, time for a serial build 
of all the examples is 14s with modules and 38s with #includes.  That 
excludes the module build time.  I also had to move the most common 
template instances into module implementation files (these are hidden 
behind non-template factory functions) to see the speedup.

3) Tooling support for CI just plain sucks, I ended up writing batch and 
shell scripts just to test this.

4) It's ever so very flaky.  For example I simply could not figure out 
the correct way to #include third party headers in module implementation 
units - this is not helped by different tutorials giving different 
answers to this issue.  The only method that clang and msvc seem to 
agree on is:

module;

#include "third_Party.hpp"

module boost.regex;Â  // note not exported in implementation unit.

I'm actually reasonably sure this is not legal code outside of the 
module definition.

5) Using the regex is an all or nothing affair, with msvc this works:

import boost.regex;

int main(int argc)
{
 Â Â  boost::regex e;
 Â Â  return 0;
}

as does:

import std;

import boost.regex;

int main(int argc)
{
 Â Â  boost::regex e;
 Â Â  return 0;
}

But this blows up:

#include <iostream>

import boost.regex;

int main(int argc)
{
 Â Â  boost::regex e;
 Â Â  return 0;
}

This makes it literally impossible to port the main test program to test 
the module because it relies on third party headers which pull in std 
lib headers (ie other boost headers).

I'm probably halting this for now... it's been a somewhat fun 
experiment, but I feel we need a few more compiler releases and better 
tooling before this can move forward, oh and a well written tutorial on 
modules which isn't full of misleading information.

John.