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Does the line template class Derived; exist in a header-file, which again is included in multiple source-files?

In that case the, vtable and methods of class Derived will exist in multiple object-files. And the linker has to figure out what to do with those multiple copies.

How the compiler and linker is supposed resolve this according to the c++ standard, i am not sure of. But typically I don't care since the copies should normally look the same.

But to avoid this issue, you should put extern template class Derived; in the header file, and template class Derived; in exactly 1 compile unit (aka. source-file)

EDIT (to reflect your split of the code into "file.hpp" and "file.cpp"):

I have played a little bit with clang-6 (I that is the latest version I have)

To me the warning is of the type "If you do X, Y will happen". But it doesn't mean y has happened.

In this case Y is multiple vtables and that will only happen if you put template class Derived; in multiple source files, which you dont't do.

The warning gets triggered for each template class Derived; in your sources, so if you only see one warning, there will be only one vtable.

But there is a way to get rid of the warning: Do not have explicit instantiation, and rely on the compiler to instantiate the class implicitly.

For that you have to put all of your template definition in the header file. So move the definitions:

Gmail users would be able to install the add-in only if the add-in is successfully validated for the gmail account as part of add-in validation. Do you know if your add-in passed the Gmail validation? Are you able to see your add-in in the outlook Mac add-in store on a gmail account?

In your AudioManager class you could implement a callback (closure) like:

var isAudioStopped: ((Bool) -> Void)?

And you can call it in your extension, in the delegate method: