c++ - derived assignment operator calling the one from a base

In the best rated answer to the question from this link, I don't understand how the derived assignment operator calls the assignment operator from the base class, i.e., this in this part of the code:

Derived& operator=(const Derived& d)
{
    Base::operator=(d);
    additional_ = d.additional_;
    return *this;
}

what does

Base::operator=(d)

mean? What does it do? Cheers!

Answer

It's just a call to a member function on the current object. The current object inherited a function whose fully-qualified name is Base::operator=, and you can call it just like any other non-static member function.