Undefined symbol to destructor only on MacOS

Question

In a library i'm working on, I have something like this template class in a header file:

template <int something>
class Base {
public:
    virtual ~Base();  // Implemented in c++ file
}

class Derived : public Base<100> {
public:
    ~Derived() override = default();
}

The destructor is implemented like so:

template <int something>
Base<something>::~Base() {
    destroyBase();
}

This code is compiled as a static library.

Then, I have this code in a sample:

{
   Derived x;
   x.doSomething();
} // X gets destroyed

I compile the executable and link the static library. This works fine on Ubuntu, but when I try it on mac, the linking fails with

Undefined symbols for architecture x86_64:

  "Base<100>::~Base()", referenced from:

      _main in my_sample.cpp.o

In both cases, I'm using clang and compiling with the exact same cmake settings. All the other symbols defined in the cpp file are defined. This is the only undefined symbol. What is happening?

Answer 1

As Sebastian Hoffmann said, templates must be implemented in the header file and not a cpp file, otherwise it won't be visible. That's why I was getting undefined symbols.

the build on mac now works when I implemented everything in the header file. Still, this doesn't explain why this only happens on Mac but works perfectly on ubuntu. Also, Why only the destructor? I had other member functions of Base implemented in a cpp file and they linked just fine!

update: I think the reason only the destructor fails to link is that it is the only one that the client app directly references. The constructor is only referenced through the derived class constructor, and other member functions and operator= and stuff are never actually called. The linker error makes sense from a c++ standpoint, but the difference between Mac and ubuntu is puzzling.

note: I'm using travis ci for builds and this is actually my personal hobby project at https://github.com/RotartsiORG/StoneMason (it's not very good)