How to export functions and classes from module not using `export` keyword? - c++

Question

I'm investigating an opportunity of using C++ Modules TS in my pet project. One of the important use cases for me is wrapping of legacy headers.
Suppose I have a header file with some functions and classes, std.io.ixx:
int f(int x)
{
return 2 + x;
}
According to this article, I compile the module using the following command:
cl /c /experimental:module /module:name std.io /module:export std.io.ixx
Which gives me a new file std.io.ifc. Then I use this module in another source file, main.cxx:
import std.io;
int main()
{
f(5);
}
Which is compiled with the following command:
cl /c /experimental:module main.cxx
The compilation gives me a following error:
main.cxx(5): error C3861: 'f': identifier not found
So, as we can see, the identifiers from the module were not exported. I could fix this by manually adding export keyword before each identifier I want to export, but this is impossible for the use case of wrapping the legacy headers.
What I'm doing wrong? How to export all possible identifiers from a header?

Answer 1

I believe there are two things wrong:
The module name must not start with std.. When I attempted to do that, I got the error
error C3674: could not find standard library module 'std.io'
This error might not appear if you haven't installed the Standard Library Modules component for Visual Studio. I'm not sure about this, though.
In the blog post you linked to, there's this note:
Note that you currently have to include your header in a .cpp file (or rename your header) because of a limitation in our compiler’s file handling.
That indeed appears to be the case because when I tried with the extension ixx, I got the same error as you.
But after fixing both of the above issues, it worked fine.