Compiling C++20 program that uses modules with Ubuntu 22.04

Question

I'm trying to run a simple program:

import <iostream>;
int main()
{
std::cout << "Hello, World!" << std::endl;
return 0;
}

I've checked, and supposedly I've already have the newest version of gcc. build-essential is already the newest version (12.9ubuntu3).

I've tried running:

g++ -std=gnu++20 -o hello hello.cpp 

or

gcc -std=c++20 -o hello hello.cpp 

But both of them give me

hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
    1 | import <iostream>;
      |         ^~~~~~~~
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:9: error: ‘iostream’ was not declared in this scope
hello.cpp:1:1: error: ‘import’ does not name a type
    1 | import <iostream>;
      | ^~~~~~
hello.cpp:1:1: note: C++20 ‘import’ only available with ‘-fmodules-ts’, which is not yet enabled with ‘-std=c++20’
hello.cpp: In function ‘int main()’:
hello.cpp:4:6: error: ‘cout’ is not a member of ‘std’
    4 | std::cout << "Hello, World!" << std::endl;
      |      ^~~~
hello.cpp:1:1: note: ‘std::cout’ is defined in header ‘<iostream>’; did you forget to ‘#include <iostream>’?
  +++ |+#include <iostream>
    1 | import <iostream>;
hello.cpp:4:38: error: ‘endl’ is not a member of ‘std’
    4 | std::cout << "Hello, World!" << std::endl;
      |                                      ^~~~
hello.cpp:1:1: note: ‘std::endl’ is defined in header ‘<ostream>’; did you forget to ‘#include <ostream>’?
  +++ |+#include <ostream>
    1 | import <iostream>;

So then I ran: gcc -std=c++20 -fmodules-ts hello.cpp

Now I get

In module imported at hello.cpp:1:1:
/usr/include/c++/11/iostream: error: failed to read compiled module: No such file or directory
/usr/include/c++/11/iostream: note: compiled module file is ‘gcm.cache/./usr/include/c++/11/iostream.gcm’
/usr/include/c++/11/iostream: note: imports must be built before being imported
/usr/include/c++/11/iostream: fatal error: returning to the gate for a mechanical issue
compilation terminated.

I'm currently stuck here...

Answer 1

Module support in g++ is not complete as of the date of posting. In particular,

Standard Library Header Units

The Standard Library is not provided as importable header units. If you want to import such units, you must explicitly build them first. If you do not do this with care, you may have multiple declarations, which the module machinery must merge—compiler resource usage can be affected by how you partition header files into header units.

You can build the iostream module using

g++ -std=c++20 -fmodules-ts -xc++-system-header iostream

This creates a gcm.cache directory in the current directory, with content like

$ tree gcm.cache/
gcm.cache/
└── usr
    └── include
        └── c++
            └── 11
                └── iostream.gcm

4 directories, 1 file

(I am using the default gcc/g++ 11.2.0-19ubuntu1 that ships with Ubuntu 22.04).

Then you may build your hello.cpp using the -fmodules-ts compiler flag:

$ g++ -std=gnu++20 -fmodules-ts -o hello hello.cpp
$ 
$ ./hello
Hello, World!

References:

• GCC documentation: 3.23 C++ Modules

• C++20 Modules — Complete Guide