VC++ experimental modules does not work

Question

I am trying the experimental implementation for modules in Visual Studio 2017, version 15.4.4. I followed the instructions described here https://blogs.msdn.microsoft.com/vcblog/2017/05/05/cpp-modules-in-visual-studio-2017/. I was able to make this run pretty quickly in a console application.

import std.core;
int main()
{
   std::cout << "Hello modules!" << std::endl;
   return 0;
}

Importing and using the available standard modules is not a problem (as far as I tried to far).

However, when I define my own module, nothing works. I added a file system.ixx (item type C/C++ compiler), with the following content:

import std.core;
export import system.io;

export struct console
{
   void write(std::string_view text) { std::cout << text; }
   void write_line(std::string_view text) { std::cout << text << std::endl; }   
};

when I add import system.io to main.cpp

import std.core;
import system.io;

int main()
{
   std::cout << "Hello modules!" << std::endl;
   return 0;
}

I get the following errors:

1>system.ixx
1>system.ixx(2): error C2230: could not find module 'system.io'
1>main.cpp
1>main.cpp(2): error C2230: could not find module 'system.io'

I also added /module:reference system.io.idf to the compiler options, but there is no system.io.idf file generated out of system.ixx.

I know this is experimental and has lots of issues, but I was wondering was should I do to make this simple thing work.

user_12089150 · Answer

In system.ixx, try changing export import system.io to export module system.io. You also want to make sure that the export module ... declaration appears at the top of the file. So system.ixx becomes:

export module system.io;
import std.core;

export struct console
{
   void write(std::string_view text) { std::cout << text; }
   void write_line(std::string_view text) { std::cout << text << std::endl; }   
};

VC++ experimental modules does not work

1 Answers