I figured out my problem... I forgot to include the widget in the .lib.pro file...

I meant that you can write as an inline (in the class declaration) or not, but the compiler can never inline the code (as it's a call trough the virtual table). And of course virtual and inline are mutually exclusive as modifiers in the declaration.

Basically:

class A { virtual void a() {} }

is the same as

class A { virtual void a(); } void A::a() { }

@koahnig

So basically all is broadened to hopefully all compilers and OSs.

It should be. To be honest, this default exporting behavior is one of the more suspicious decisions in Linux. Windows' way of enforcing symbols to be hidden by default (like we have with classes' members) is much more logical. I actually define two macros when making shared libraries - one that is for the export/import and one that forces symbols not explicitly exported to be hidden on Linux, thus getting a similar binary interface on all platforms; like done here.

Kind regards.