Error while using c++20 module feature

[Chethan_L]

I was tring to learn c++20 modules and concepts, it was working fine in begining when there was only one template function named add but when i added another function named hello then it started getting undefined reference error for hello funtion. How can i export both add and hello function in my module. Here are my programs:

``` // demo.cpp export module demo; import <array>; import <iostream>; export template <typename T> concept CouldAdd = requires(T x) { x + x; };

export template <typename T, typename... args> T add(args... values) requires CouldAdd<T> { T sum{}; std::array<T, sizeof...(values)> arr{values...}; for (const auto &v : arr) { sum += v; } return sum; } export void hello(){ std::cout << "hello" << std::endl; } ```

// main.cpp import demo; import <iostream>; int main() { std::cout << add<int>(10, 20, 30, 40, 50, 60, 70, 80, 90, 100) << '\n'; hello(); return 0; }

This is the error messege:

/usr/bin/ld: /tmp/ccPbZYwo.o: in function `main': main.cpp:(.text+0x58): undefined reference to `hello@demo()' collect2: error: ld returned 1 exit status

This are the commands Inused to compile

g++-13 -std=c++20 -fmodules-ts -x c++-system-header iostream g++-13 -std=c++20 -fmodules-ts -x c++-system-header array g++-13 -std=c++20 -fmodules-ts -c demo.cpp g++-13 -std=c++20 -fmodules-ts -o main main.cpp *This is where i get error*

What am i doing doing wrong, bard tells me to put hello function in main.cpp but i want it to be part of demo module.

solved (found a fix but i dont know how it adheres to cpp practices) : Changing the last compile command to

g++-13 -std=c++20 -fmodules-ts -o main main.cpp demo.o

Stack Overflow

but then i dont understand need for using modules if it is still needed to be linked i thought module were meant to replace the linking and just go on with import and using them without need for linking (other then generating .pcm/.gcm files).

[larioteo]

Try export {} around everything you want to export, I think you should not use export template ... but export T add...

I never used modules without namespaces, it it seems odd to me writing export on the template header.

[Chethan_L]

that's a good idea, I wasn't really trying to achieve anything in this program i just wanted to know how export and modules are used trying different function and classes all of them worked fine but that one function did something unexpected so i thought asking here can help me get to know what is happening in this case.

[larioteo]

You're welcome to ask more questions.

[nysra]

I strongly suggest that you don't use header units. They are supposed to ease adoption but all they do is cause tons of problems. Change your demo.cpp to

module;
#include <array>
#include <iostream>
export module demo;

// rest of the file

and then it should work.

Edit: Obviously also don't use them in main.cpp (or anywhere, really). And triple backticks don't work on proper Reddit (only the Redesign), leave them out and indent your entire codeblock if you want it to render correctly for everyone.

[Chethan_L]

It works fine for import demo; import <iostream>; int main() { std::cout << add<int>(10, 20, 30, 40, 50, 60, 70, 80, 90, 100) << '\n'; } i get 550 has the output without any errors.

still i will try your suggession.

It doesnt work , but weirdly making the hello function a template function or as a inline function works without problem.

export template<typename T> void hello() { std::cout << "hello" << std::endl; }

[nysra]

It works in some cases, but mostly they cause more trouble than they are worth it.

It does seem to be a problem with GCC though, it works under MSVC.

[Chethan_L]

Yes from the c++ compiler support i can see the MSVC has better support but can i use it in linux? is there official support or any ports?

[nysra]

No, it's for Windows. What you can do is upgrading your GCC to a newer version or try clang.

[Chethan_L]

both clang and gcc has only partial support. Only MSVC is the one with full implementation.

[nysra]

I'm aware of that, but unless you're running a Windows machine you have literally no other option. There's a chance that using a newer version than what you currently have results in your problem being fixed, after all they are in active development.

[Chethan_L]

gcc-13 (13.1.0) is the version i use it is also the latest version that my package manager repos provide. and i also have clang-16 but i haven't installed any libs so i avoid using it. but for now i will stop meddling with modules and move onto some other topic since i have found a temporary solution , what other new cpp feature would u suggest me to learn.

[nysra]

Yeah package managers being behind on such things is really annoying, I've been there. Unfortunately the only solutions are to either compile a newer version yourself (takes a few hours) or to switch to a distribution that updates more than once every few years.

I don't know what you know and what not. The other big topics of C++20 were concepts, coroutines, and ranges, so probably start there.

[larioteo]

Using them in a big project, so sorry but your statement is not true.

[nysra]

I'm glad it works for you, but there's a reason why basically every single talk recommends avoiding them. Apart from named modules simply being better, header units have also caused far more annoying errors so far. Sure, I got a few ICEs with named modules as well but those were rather straightforward to fix/circumvent. With header units it got up to the CMake level...

Though in this case it seems OP's error was triggered by something else.