C++20 modules with gnu g++

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C++20 - modules with gnu g++

this page will describe the gnu\g++ way of developing with modules with c++20.


while reading books on latest C++, I found I could not compile any of the examples straight away, making a bit difficult to follow and practice as I read along.

many of the resources I tried on the net or paper to compile and expand on the examples were not complete or very simple, rendering the examples on the books hard to follow.

here I leave an example of my own which is simple enough but can scale to more elaborated ones as I break implementation of module components into many files.
For the time being, all module definitions still remain on a single file.

Current C++20 supported features by gnu g++

check this website for the support status of c++20 features by gnu\g++.

Installing gnu g++

I am using OpenSuSE Tumbleweed, if you have a different one, the goal is to install gcc11 with c++ support. Check your ow distro as usually non rolling distribution require some additional steps before adding some latest versions of software.

run the below to get latest gcc11 in your machine. it will install the latest (for now) version of gcc with c++ support along with the many corresponding libraries and headers.

# on a machine running OpenSuSE tumbleweed
user@hostname:~$ sudo zypper in gcc11 gcc11-c++ gcc11-info 


to demonstrate simple, yet multi_file “hello modules” intro program featuring c++20 modules I will do the following:

  • create a main file that makes use of a module
  • create a module definition file
    • with a class
    • with an isolated function
  • create a module implementation file for the class defined on the previous module file
  • create a module implementation file for the method defined on the previous module file

The example will be a simple Person class with a function that operates on a Person.


given that support for c++20 modules is a still a little green, g++ can get confused by a few file extensions used for other compilers or on some books.

use the same extensions as in this example, and you’ll be ok. I you change .cpp for a .cppm for example, you will get an error message similar to the one below.

/usr/lib64/gcc/x86_64-suse-linux/11/../../../../x86_64-suse-linux/bin/ld:person_module.cppm: file format not recognized;
treating as linker script
/usr/lib64/gcc/x86_64-suse-linux/11/../../../../x86_64-suse-linux/bin/ld:person_module.cppm:1: syntax error
collect2: error: ld returned 1 exit status

naming modules can be a source of confusion on gnu g++ as a few books using Clang or MS compilers add extensions such as .cppm.
a note about not supporting new file suffixes, can be found here on the g++ online docs

No new source file suffixes are required or supported. If you wish to use a non-standard suffix (See Overall Options), you also need to provide a -x c++ option too.2

you can tell g++ that this or that extension is a file of a given type, check the above link for a hint, (use -x to explicitly specify the language).

given the extra options just to use .cppm instead of .cpp, I am just sticking with .cpp.

system headers and modules.

the short story is that they don’t play very well out of the box.

thou the standard is out, and the compilers are supporting more and more features, you need -for the moment at least-, a previous step to compiling you program, which is compiling the system headers themselves into modules, so to use them with the new modules’ syntax.

to achieve this, say you want to use <iostream> as a module import instead of as a library #include, you need this step previous to compilation.

user@hostname:~$ g++-11 -std=c++20 -fmodules-ts -c -x c++-system-header iostream

what the above is doing is telling g++-11 that:

  • -std=c++20: you want to use c++20 standard
  • -fmodules-ts: you are using the feature modules technical specification
  • -c: you don’t want to run the linker, just compile
  • -x c++-system-header iostream: specifies explicitly what the next files are, the c++ system header iostream in this case

that step will place the iostream header, now a module, inside the gcm.cache folder local to where u ran the command. If u navigate the folder until finding the iostream module, iostream.gcm, you will see that it respects the include path of the original header, gcm.cache/usr/include/c++/11/

if you don’t care for this first step, just don’t precompile the system headers into modules and keep using #include <iostream>.

on the next examples, where u see import <iostream>; just replace it with #include <iostream as usual c++.

all this will become easier when using compilation tools such as make, as the step can be included in a makefile, and we forget about them


import <iostream>;

export module data_type;

// by exporting the namespace we export everything under it, or else we need to export
// everything explicitely
export namespace data_type
    // a class in a module under a namespace
    class Person
        int m_age;
        std::string_view const m_name;

        Person(int age, std::string_view const &name);
        Person(int age);
        void printPerson();

        // getter as example...
        // can also add the setters if desired
        int getAge();
        std::string_view const &getName();

    // a standalone function in a module under a namespace
    void print_person_external(Person &person);

the module defines under namespace data_type, a Person class that has 2 private members, 2 public constructors, and 2 public getters plus a standalone function that will operate on a Person.

we have to declare at the top of the file the module keyword and after the includes we state that we want to export this module with the name data_type

we also need to tell the compiler what do we want to export, by prepending a declaration with export. In this case we are declaring that we want to export a namespace, and by doing that everything in that namespace is exported (the class Person and also the function print_person_external)

If we had placed the function outside the namespace, it would not have been exported. If it is the case you want something outside the namespace and still exported as part of the module, just prepend the declaration with export and it will also be exported.

the order is important…

if you place the #include <iostream> above the module; and try to compile the example, you will get the following error:

person_module.cpp:2:1: error: module-declaration only permitted as first declaration,
or ending a global module fragment
    2 | module;
      | ^~~~~~
person_module.cpp:4:8: error: module-declaration only permitted as first declaration,
or ending a global module fragment
    4 | export module data_type;
      |        ^~~~~~
person_module.cpp:7:1: error: ‘export’ may only occur after a module interface declaration
    7 | export namespace data_type
      | ^~~~~~


import <iostream>;

module data_type;

namespace data_type
  // implementations of the person Class here
  Person::Person(int age, std::string_view const &name) : m_age{age}, m_name{name} {};
  Person::Person(int age) : m_age{age}, m_name{"default"} {};

  void Person::printPerson()
    std::cout << "person name is: " << m_name << std::endl;
    std::cout << "person age is: " << m_age << std::endl;

  int Person::getAge()
    return m_age;

  std::string_view const &Person::getName()
    return m_name;

this file contains the implementation of the class constructors and methods.

note how we still use module; at the top of the file, followed by #include and the module name that we are implementing WITHOUT the export keyword, as we are not exporting anything.

we start with the namespace and inside the implementation of both constructors, the internal printPerson() function and both getters. This is just like any other implementation.

note how we are referring to the private members m_age and m_name without problems as they are visible by Person itself.

the order is still important…

as we’ve seen before order matters… If you would have placed the #include <iostream> after module data_type; you will get the following error when trying to compile:

In file included from /usr/include/c++/11/bits/exception_ptr.h:38,
                 from /usr/include/c++/11/exception:147,
                 from /usr/include/c++/11/ios:39,
                 from /usr/include/c++/11/ostream:38,
                 from /usr/include/c++/11/iostream:39,
                 from person.cpp:3:
/usr/include/c++/11/bits/cxxabi_init_exception.h:52:9: error: cannot declare ‘struct std::type_info’ in a different module
   52 |   class type_info;
      |         ^~~~~~~~~
<built-in>: note: declared here
In file included from /usr/include/c++/11/bits/exception_ptr.h:39,
                 from /usr/include/c++/11/exception:147,
                 from /usr/include/c++/11/ios:39,
                 from /usr/include/c++/11/ostream:38,
                 from /usr/include/c++/11/iostream:39,
                 from person.cpp:3:
/usr/include/c++/11/typeinfo:88:9: error: cannot declare ‘struct std::type_info’ in a different module
   88 |   class type_info
      |         ^~~~~~~~~
<built-in>: note: declared here
In file included from /usr/include/c++/11/bits/exception_ptr.h:40,
                 from /usr/include/c++/11/exception:147,
                 from /usr/include/c++/11/ios:39,
                 from /usr/include/c++/11/ostream:38,
                 from /usr/include/c++/11/iostream:39,
                 from person.cpp:3:
/usr/include/c++/11/new:89:27: error: cannot define ‘enum class std::align_val_t’ in different module
   89 |   enum class align_val_t: size_t {};
      |                           ^~~~~~
<built-in>: note: declared here
/usr/include/c++/11/new:89: confused by earlier errors, bailing out


import <iostream>;

module data_type;

namespace data_type
  //implementation of the functions here
  void print_person_external(Person &person)
    std::cout << "person name is: " << person.getName() << std::endl;
    std::cout << "person age is: " << person.getAge() << std::endl;

same as with the Person class implementation, we state this will be part of module data_type.

I just did this on a different file to get my head around bigger projects and how to keep possible big codebases tidy and separated. Even if it doesn’t make much sense on this small program, it illustrates how breaking implementation of a module export can be achieved.


import data_type;
import <iostream>;

using namespace data_type;
using namespace std;

int main(void)
  cout << "------\n";
  cout << "------ printing person details with INTERNAL to class function\n\n";

  cout << "------ person created with single arg ctor\n";
  Person person{1};
  cout << "person age: " << person.getAge() << endl;

  cout << "\n------\n";
  cout << "------ person created with 2 args ctor\n";
  Person person2{2, "test"};
  cout << "person2 age: " << person2.getAge() << endl;
  cout << "person2 name: " << person2.getName() << endl;

  cout << "\n------\n";
  cout << "------ printing person details with EXTERNAL to class function\n\n";

  cout << "-----\n";

  return 0;

this is a ceremonial main.cpp to illustrate the c++20 modules feature, note how we do not #include a module, but we import it.

after some cout to get our bearings into the console print, we use all the functionality declared and implemented on every file:

  • multiple constructors
  • multiple getters
  • internal (to the class) print person function
  • external (to the class) print person function


we have 4 files:

  • main.cpp (main)
  • person_module.cpp (module)
  • person.cpp (implementation of Person::functions)
  • person_func.cpp (implementation of isolated function)

to compile this you need to call g++ v11 indicating that you want to use c++20, and the modules features like this.

# remember this step to compile the headers into modules if you haven't done it before
user@hostname:~$ g++-11 -std=c++20 -fmodules-ts  -c -x c++-system-header iostream

# now the command to compile the program itself...
user@hostname:~$ g++-11 -std=c++20 -fmodules-ts -o person person_module.cpp person.cpp person_func.cpp main.cpp

the shell will not respond with anything, indicating everything went ok. To execute this little demo just hit ./person on your console to run it.

# To execute this little demo just type `./person` and press enter
user@hostname:~$ ./person 

# you should see the following output

------ printing person details with INTERNAL to class function

------ person created with single arg ctor
person age: 1
person name is: default
person age is: 1

------ person created with 2 args ctor
person2 age: 2
person2 name: test
person name is: test
person age is: 2

------ printing person details with EXTERNAL to class function

person name is: default
person age is: 1