How to use c++ 20 modules in shared libraries ?

Question

Hello everyone,

I am currently working on a test-project where I want to use shared libraries and I would like to leverage the new C++20 modules feature. However, I am having some difficulty understanding how to properly set up my code to use C++20 modules between these shared libraries.

I have already set my compiler to use the C++20 standard, but I am unsure of the changes I need to make in my code and build system to use modules with shared libraries.

// test.modules.ixx

export module test.modules;

export void myFunction();

export class Test
{
public:
  Test();
};

// test.modules.cpp

module;

#include <iostream>

module test.modules;


void myFunction()
{
  std::cout << "hello from module";
}

Test::Test()
{
  std::cout << "init";
}

Command line options

/JMC /experimental:module /permissive- /ifcOutput "x64\Debug\" /GS /W3 /Zc:wchar_t /ZI /Gm- /Od /sdl /Fd"x64\Debug\vc143.pdb" /Zc:inline /fp:precise /D "_DEBUG" /D "TESTLIB_EXPORTS" /D "_WINDOWS" /D "_USRDLL" /D "MATHLIBRARY_EXPORTS" /D "_WINDLL" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /RTC1 /Gd /MDd /std:c++latest /FC /Fa"x64\Debug\" /EHsc /nologo /Fo"x64\Debug\" /Fp"x64\Debug\testLib.pch" /diagnostics:column 

When importing this into another project i get following errors:

// My Executable
import test.modules;

int main()
{
    std::cout << "Hello World!\n";
    myFunction();
    Test();
}

error C2230: could not find module 'test.modules'
error C3861: 'myFunction': identifier not found
error C3861: 'Test': identifier not found

Here are some specific questions I have:

1. How should I structure my code to define and import modules in different shared libraries?
2. Are there any specific compiler flags I need to use to enable modules?
3. Are there any known issues or limitations I should be aware of when using C++20 modules with shared libraries?

Any examples, resources, or advice would be greatly appreciated. Thank you in advance for your help!

Thanks

Answer 1

Hi, Michael Brunner •

If module is used in DLL, you need to use _declspec(dllexport) to export the symbol.

1. In module.ixx:

export _declspec(dllexport) void myFunction();
export  class _declspec(dllexport) Test
{
......
}

2. After building the DLL, find the test.modules.ifc file (By default, it is in the same directory as the project's obj file).
3. In console project:

Project properties -> C++ -> Command Line opition -> Add /reference "path to DLL project\x64\Debug\test.modules.ifc"

4. In addition, add #include<iostream> in console project.

Best regards,

Minxin Yu

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Answer 2

Follow-Up Question on C++20 Modules in Shared Libraries:

Hello,

I’ve been exploring the use of C++20 modules in shared libraries and I understand the typical approach involves providing an .ifc file. However, I’m curious if there’s a way to utilize these modules without distributing the .ifc file. Could you provide guidance on how to use a DLL module in such a scenario?

• Alternative Methods: Are there alternative methods to import modules without an .ifc file?
• DLL Usage: How can one use the DLL module without the .ifc file in another project?
• Build System Configuration: What changes are needed in the build system to support this usage?

Any insights or examples would be greatly appreciated

Thank you!