feat: Add C++ modules support by mikomikotaishi · Pull Request #1530 · brainboxdotcc/DPP

mikomikotaishi · 2025-12-27T11:45:58Z

This pull request adds support for C++20 modules.

In order to accomplish this, some constants were changed to using external linkage.

~~I have also updated nlohmann/json to version 3.12.0, as the older version declares some symbols as internal.~~ Updating nlohmann/json has been moved to #1596.

Code change checklist

I have ensured that all methods and functions are fully documented using doxygen style comments.
My code follows the coding style guide.
I tested that my change works before raising the PR.
I have ensured that I did not break any existing API calls.
I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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Mishura4

Thanks for the PR! Good stuff. Just gonna prefer the approach with including dpp.h in the module for maintainability, as discussed in https://discord.com/channels/825407338755653642/825411104208977952/1454456223952011427

braindigitalis

some module related unit tests would be good. not repeating a thousand lines of code in dpp.cppm is an absolute must, as @Mishura4 also said.

mikomikotaishi · 2025-12-27T20:21:50Z

I've looked at the src/unittest/ files but I'm not particularly familiar with how you've set up the unit testing framework. Are you just looking to run those same tests under import dpp; rather than #include <dpp/dpp.h>?

braindigitalis · 2025-12-27T22:48:21Z

I think that in this case you might want to make a new .cpp under the unittests folder which imports dpp instead of #including it. it can have some test functions that are exported and called via standard approaches from the main unittest file.

mikomikotaishi · 2025-12-28T09:13:47Z

I think this should fix the errors we're encountering. It's with the build system configuration

braindigitalis · 2025-12-28T13:18:42Z

you cant just do -DDPP_MODULES=ON for the entire CI matrix. Some CI's use older g++ (which we support) and some use C++17.

mikomikotaishi · 2025-12-28T17:25:19Z

Then is it possible to disable the modules unit tests for C++17 CI tests?

mikomikotaishi · 2025-12-29T01:01:20Z

@braindigitalis I think the modules CI should just be in a totally separate CI. It requires Ninja, as CMake can't build modules with Makefiles.

braindigitalis · 2025-12-29T08:22:13Z

unit tests only run in one g++, which is g++-12. This is the one which is packaged. we cant and dont run unit tests on them all, as the unit tests connect to discord and we cant do this concurrently.

Jaskowicz1 · 2025-12-29T12:22:44Z

Updated Title to match code standards

braindigitalis · 2026-07-01T01:28:59Z

any chance we can trim it down so that its easier to review? perhaps by not introducing ninja as a dependency of docs build, not changing its compiler to clang, and keeping changes to a minimum in the cmakelists etc? right now there are 53 changed files.

braindigitalis

left some thoughts :-) thanks for the continued effort on this!

braindigitalis · 2026-07-01T01:32:44Z

 	});

 	/* The interaction create event is fired when someone issues your commands */
-	bot.on_slashcommand([&bot](const dpp::slashcommand_t & event) {


do we need to change all the examples, it makes the PR much harder to review, so many files to go through.

I've moved these changes into a separate PR, #1604

braindigitalis · 2026-07-01T01:36:04Z

-				bytes.a
-				tls_syntax.a
-				hpke.a
+				$<BUILD_INTERFACE:mlspp.a>


really really not liking that we changed this. it took a LOT of effort to make this always build everywhere. with static and dynamic linking, on a ton of different systems. some of which CI doesnt test (e.g. static builds)

$<BUILD_INTERFACE:> is build-time transparent, during build it expands into mlspp.a and produces the same linker command.

then if it does the same why change it?

@braindigitalis BUILD_INTERFACE only expands when building the library artifacts ; it's usually as a counterpart to INSTALL_INTERFACE which only expands when bringing the library from an install directory

Up until modules that was functionally the_ same as PRIVATE / PUBLIC but with modules actually PRIVATE is included when bringing module files (as they are translation units that need to be built privately into a public binary artifact, like .cpp files), even when linking an installed library. So BUILD_INTERFACE prevents this and is only used when building the library itself.

So adding BUILD_INTERFACE is correct here, even if I am not sure the author knew why.

braindigitalis · 2026-07-01T01:37:23Z

@@ -0,0 +1,141 @@
+/************************************************************************************


this doesnt actually test modules, just verifies they can be linked. but, i guess, theres nothing else we can do to test that, as it is a compile-time thing.

mikomikotaishi · 2026-07-01T10:46:13Z

OK; will look in to this.

braindigitalis · 2026-07-01T12:32:01Z

all builds failed. nlohmann doesnt need touching in this pr any more.

braindigitalis · 2026-07-07T12:16:07Z

this file should not be picked up as changed at all in this pr. please merge master into the branch

braindigitalis · 2026-07-07T12:20:10Z

 		# Installation

 		include(GNUInstallDirs)
-		install(TARGETS dpp LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR})


why did this get removed, is it somewhere else now?

This install does still take place; it's done in cmake/CPackSetup.cmake, which has a richer install rule anyway; keeping it library/CMakeLists.txt just reinstalls the library a second time (with less information)

Mishura4

Example doesn't seem to build on Windows. Can we add it to CI somehow?
Also, please fix merge conflicts.

Mishura4 · 2026-07-18T16:32:53Z


+set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -rdynamic")
+
+# Create gcm.cache directory and symlink to the main build's dpp.gcm


Yes I can see that, but why?

Clang obtains the BMI paths through -fprebuilt-module-path which indicates where to scan, but GCC has no such option, and places its BMIs in gcm.cache/, which is why the lookup location is populated with the symlink to the actual interface.

Why is this necessary? I've not had to do this with any of my other projects

Mishura4 · 2026-07-18T16:49:48Z

-				bytes.a
-				tls_syntax.a
-				hpke.a
+				$<BUILD_INTERFACE:mlspp.a>


@braindigitalis BUILD_INTERFACE only expands when building the library artifacts ; it's usually as a counterpart to INSTALL_INTERFACE which only expands when bringing the library from an install directory

Up until modules that was functionally the_ same as PRIVATE / PUBLIC but with modules actually PRIVATE is included when bringing module files (as they are translation units that need to be built privately into a public binary artifact, like .cpp files), even when linking an installed library. So BUILD_INTERFACE prevents this and is only used when building the library itself.

So adding BUILD_INTERFACE is correct here, even if I am not sure the author knew why.

Jaskowicz1

A couple comments, nothing to really reject for though

Jaskowicz1 · 2026-07-19T11:23:38Z

@@ -0,0 +1 @@
+\warning D++ modules are currently only supported by D++ on g++ 15, clang/LLVM 17, and MSVC 17.6 or above. Additionally, your program has to support C++20.


Sorry to nitpick, could we word this to be D++ modules are currently experimental and are only supported by D++ on g++ 15, clang/LLVM 17, and MSVC 17.6 or above. Additionally, your program has to support C++20.

Jaskowicz1 · 2026-07-19T11:30:53Z

@@ -1 +1 @@
-\warning D++ Coroutines are currently only supported by D++ on g++ 13, clang/LLVM 14, and MSVC 19.37 or above. Additionally, your program has to support C++20.
+\warning D++ coroutines are currently only supported by D++ on g++ 13, clang/LLVM 14, and MSVC 19.37 or above. Additionally, your program has to support C++20.


I would remove this change, doesn't really match the capitalisation across the board but also better to keep changes to a minimum

Jaskowicz1 · 2026-07-19T11:33:38Z

+          DONT_RUN_VCPKG: true
+
+      - name: Build module example
+        run: cmake --build build --target using_modules_example --config Release --parallel 2


does this target exist? I can't see it anywhere in this PR. If it does exist, it's not being built for Windows.

This is causing CI to fail

I asked for examples to be built in CI but yes the CI needs to be fixed

Yeah i'm happy with that, it's just the fact that the using_modules_example target doesn't look like it exists

github-actions Bot added documentation Improvements or additions to documentation build Issue or Pull Request related to the build process code Improvements or additions to code. labels Dec 27, 2025

Jaskowicz1 requested review from Jaskowicz1, Mishura4 and braindigitalis December 27, 2025 11:57

Jaskowicz1 added the enhancement New feature or request label Dec 27, 2025

mikomikotaishi force-pushed the dev branch 3 times, most recently from e91eaa8 to 121eb9b Compare December 27, 2025 12:04

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Jaskowicz1 changed the title ~~Add C++ modules support and update nlohmann::json~~ feat: Add C++ modules support and update nlohmann::json Dec 29, 2025

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mikomikotaishi changed the title ~~feat: Add C++ modules support and update nlohmann::json~~ feat: Add C++ modules support Jun 29, 2026

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This was referenced Jul 1, 2026

Fix/unused captures #1603

Closed

fix: Remove unused captures in example code #1604

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mikomikotaishi added 2 commits July 5, 2026 11:43

Merge branch 'dev' into dev

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Remove now irrelevant attributes

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Remove edits to json.hpp

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Restore removed -rdynamic flag

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Add a static-linux and install-consumer job on CI

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Merge branch 'dev' into dev

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Apply recommended docs changes

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Add a CI for building the module on Windows

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		@@ -0,0 +1,141 @@
		/************************************************************************************


		set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -rdynamic")

		# Create gcm.cache directory and symlink to the main build's dpp.gcm

		@@ -0,0 +1 @@
		\warning D++ modules are currently only supported by D++ on g++ 15, clang/LLVM 17, and MSVC 17.6 or above. Additionally, your program has to support C++20.

		@@ -1 +1 @@
		\warning D++ Coroutines are currently only supported by D++ on g++ 13, clang/LLVM 14, and MSVC 19.37 or above. Additionally, your program has to support C++20.
		\warning D++ coroutines are currently only supported by D++ on g++ 13, clang/LLVM 14, and MSVC 19.37 or above. Additionally, your program has to support C++20.

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