How to use C++20 modules in GNU g++-11

using gcc11-20210124-32.exe (Windows build from equation.com) (Warning: This is experimental!)

module definition

#include headers in the global module fragment

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

before module module_name; or import them after that:

greetings.cpp

export module greetings;
import <iostream>;  
 
export auto say_hello()
{
    std::cout << "Hello, modules!\n";   
}

module interface unit

Every module consists of exactly one module interface unit export module module_name; after which it declares which names should be exported, and eventually other module implementation units (source files) starting with module module_name; (without export).

import module

by its module name if you want to use the contents of a module

main.cpp

import greetings;
 
int main()
{
    say_hello();    
}

Prepare (std) header unit(s)

before compiling modularized source code containing e.g. import <iostream>;

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

Other options are c++-header and c++-user-header (uses #include path).

Compile program

with option -fmodules-ts in c++20 mode:

g++ greetings.cpp main.cpp -fmodules-ts -std=c++20

Done! Start program:

> a.exe
Hello, modules!

namespace cpp {}

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