Use of undeclared identifier in C++ with templates and inheritance [duplicate]

Answer 1

There are several issues:

1. Missing semicolons after class definitions.
2. Missing type for doSomething method declaration/definition.
3. Missing type for definition of addStuff method.

After fixing that it seems to work.

Edit: As you have fixed the syntax errors and it still does not work. As others have suggested your compiler may require you to call the addStuff method with this-> prefix:

this->addStuff(thingy);