Discuss: c++ modules in b2 build

[fasxmut]

The gcc 16.0 I built from gcc git repo has improved c++ modules support greatly. I see b2 build library property can not manage the module unit dependencies correctly. And I see the obj rule and dependency property together can manage the module unit dependencies, just need more b2 jamfile code. The following code can work correctly. Is there a more elegant and better way to manage c++ modules, how does b2 build support c++ modules better.

jamroot

# This jamroot is written for gcc compiler only, requires at least gcc 15.0/16.0.

path-constant prjdir : . ;

cxxstd = 26 ;

common-cxxflags = -fmodules-ts -std=c++$(cxxstd) ;

###########################################################################
# section: generate gcm.cache/std.gcm, which is required by "import std;"

std-gcm-cache = $(prjdir)/gcm.cache/std.gcm ;

make $(std-gcm-cache) : : @make-std-gcm-cache ;

actions make-std-gcm-cache
{
	g++ $(common-cxxflags) -fmodule-only -fsearch-include-path bits/std.cc -c
	exit
}

alias std-gcm : $(std-gcm-cache) ;

###########################################################################

project
:
	requirements
		<cxxflags>$(common-cxxflags)
;

###########################################################################
# section: generate libmy-cpp.so, obj dependencies

obj
	obj-primary
:
	primary.cpp
:
	<dependency>std-gcm
;

obj
	obj-specialization
:
	specialization.cpp
:
	<dependency>obj-primary
;

obj
	obj-interface
:
	interface.cpp
:
	<dependency>obj-primary		# This line can be removed
	<dependency>obj-specialization
;

obj
	obj-specialization-src
:
	specialization-src.cpp
:
	<dependency>obj-interface
;

lib
	my-cpp
:
	obj-specialization-src
;

###########################################################################
# section: main program

exe
	main
:
	main.cpp
:
	<library>my-cpp
	<dependency>my-cpp
;

primary.cpp

export module my_module:primary;

export import std;

export namespace my_space
{
	template <typename>
	class my_class;
}

specialization.cpp

export module my_module:specialization;

import :primary;

export namespace my_space
{
	class specialization_tag {};

	template <>
	class my_class<my_space::specialization_tag>
	{
	public:
		void print() const;
	};
}

interface.cpp

export module my_module;

export import :primary;
export import :specialization;

specialization-src.cpp

module my_module;

void my_space::my_class<my_space::specialization_tag>::print() const
{
	std::cout << "Cheers, c++!" << std::endl;
}

main.cpp

import my_module;

int main()
{
	my_space::my_class<my_space::specialization_tag> object;
	object.print();
}

[grisumbras]

B2 currently doesn't support C++ modules.

As far as I can see, the script you've written will break if b2 is invoked from a directory that is different from the project root (e.g. in a subproject). For that to work a module mapper would have to be used. Although, maybe -fsearch-include-path can also work.

Also, I suspect that if you request several different configurations your script will break too.

[fasxmut]

B2 currently doesn't support C++ modules.

As far as I can see, the script you've written will break if b2 is invoked from a directory that is different from the project root (e.g. in a subproject). For that to work a module mapper would have to be used. Although, maybe -fsearch-include-path can also work.

Also, I suspect that if you request several different configurations your script will break too.

I got it. thanks.