Example with modules

main.cpp

#include <iostream>

import myfancymodule;

int main(int argc, char *argv[])
{
  int x = add(3, 4);
  std::cout << "add(3, 4) = " << x << "\n";

  int y = print_add(3, 4);
  std::cout << " = " << y << "\n";

  return 0;
}

Makefile

CXX=$(HOME)/soft/llvm/install/bin/clang++ -std=c++20

all: main

main: main.o mymod.o mymod_impl_basic.o mymod_impl_print.o
	$(CXX) -o $@ $+

main.o: main.cpp mymod.pcm
	$(CXX) -fmodule-file=myfancymodule=mymod.pcm -c -o $@ $<

# mymod
mymod.o mymod.pcm &: mymod.cppm mymod_basic.pcm mymod_print.pcm
	$(CXX) -c \
	 -fmodule-output \
	 -fmodule-file=myfancymodule:basic=mymod_basic.pcm \
	 -fmodule-file=myfancymodule:print=mymod_print.pcm $<

# mymod:basic
mymod_basic.o mymod_basic.pcm &: mymod_basic.cppm
	$(CXX) -c \
	 -fmodule-output $<

mymod_impl_basic.o: mymod_impl_basic.cpp mymod.pcm
	$(CXX) -c $<

# mymod:print
mymod_print.o mymod_print.pcm &: mymod_print.cppm
	$(CXX) -c \
	 -fmodule-output $<

mymod_impl_print.o: mymod_impl_print.cpp mymod_print.pcm mymod_basic.pcm
	$(CXX) -c \
	 -fmodule-file=myfancymodule:basic=mymod_basic.pcm $<

clean:
	rm -vf *.o *.pcm *.o.tmp main

Makefile.double-step

CXX=$(HOME)/soft/llvm/install/bin/clang++ -std=c++20

all: main

main: main.o mymod.o mymod_impl_basic.o mymod_impl_print.o
	$(CXX) -o $@ $+

main.o: main.cpp mymod.pcm
	$(CXX) -fmodule-file=myfancymodule=mymod.pcm -c -o $@ $<

# mymod
mymod.pcm: mymod.cppm mymod_basic.pcm mymod_print.pcm
	$(CXX) --precompile \
	 -fmodule-file=myfancymodule:basic=mymod_basic.pcm \
	 -fmodule-file=myfancymodule:print=mymod_print.pcm $<
mymod.o: mymod.pcm
	$(CXX) -c $<

# mymod:basic
mymod_basic.pcm: mymod_basic.cppm
	$(CXX) --precompile $<
mymod_basic.o: mymod_basic.pcm
	$(CXX) -c $<

mymod_impl_basic.o: mymod_impl_basic.cpp mymod.pcm
	$(CXX) -c $<

# mymod:print
mymod_print.pcm: mymod_print.cppm
	$(CXX) --precompile $<
mymod_print.o: mymod_print.pcm
	$(CXX) -c $<

mymod_impl_print.o: mymod_impl_print.cpp mymod_print.pcm mymod_basic.pcm
	$(CXX) -c \
	 -fmodule-file=myfancymodule:basic=mymod_basic.pcm $<

clean:
	rm -vf *.o *.pcm *.o.tmp main

mymod.cppm

/* vim: syntax=C++ */
export module myfancymodule;

export import :basic;
export import :print;

mymod_basic.cppm

/* vim: syntax=C++ */
export module myfancymodule:basic;

export int add(int a, int b);
export int sub(int a, int b);

mymod_impl_basic.cpp

module myfancymodule:basic;

int add(int a, int b) {
  return a + b;
}

int sub(int a, int b) {
  return a + b;
}

mymod_impl_print.cpp

module;
// Global module part.
#include <iostream>

module myfancymodule:print;

import :basic;

int print_add(int a, int b) {
  std::cout << "add(" << a << ", " << b << ")";
  return add(a, b);
}

int print_sub(int a, int b) {
  std::cout << "sub(" << a << ", " << b << ")";
  return sub(a, b);
}

mymod_print.cppm

/* vim: syntax=C++ */

export module myfancymodule:print;

export int print_add(int a, int b);
export int print_sub(int a, int b);

output-double-step.txt

$ make -f Makefile.double-step
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 --precompile mymod_basic.cppm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 --precompile mymod_print.cppm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 --precompile \
			 -fmodule-file=myfancymodule:basic=mymod_basic.pcm \
			 -fmodule-file=myfancymodule:print=mymod_print.pcm mymod.cppm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -fmodule-file=myfancymodule=mymod.pcm -c -o main.o main.cpp
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c mymod.pcm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c mymod_impl_basic.cpp
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c \
			 -fmodule-file=myfancymodule:basic=mymod_basic.pcm mymod_impl_print.cpp
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -o main main.o mymod.o mymod_impl_basic.o mymod_impl_print.o

$ ./main
add(3, 4) = 7
add(3, 4) = 7

output-single-step.txt

$ make
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c -fmodule-output mymod_basic.cppm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c -fmodule-output mymod_print.cppm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c \
			 -fmodule-output \
			 -fmodule-file=myfancymodule:basic=mymod_basic.pcm \
			 -fmodule-file=myfancymodule:print=mymod_print.pcm mymod.cppm
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -fmodule-file=myfancymodule=mymod.pcm -c -o main.o main.cpp
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c mymod_impl_basic.cpp
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -c \
			 -fmodule-file=myfancymodule:basic=mymod_basic.pcm mymod_impl_print.cpp
/home/roger/soft/llvm/install/bin/clang++ -std=c++20 -o main main.o mymod.o mymod_impl_basic.o mymod_impl_print.o

$ ./main 
add(3, 4) = 7
add(3, 4) = 7