Assigning class to base class, how do do it?

Question

I'm confused about assigning the base object of a derived class. Say I have a type:

class Base {
 // stuff omitted for brevity
}

and a derived class

Derived : public Base {
  // stuff omitted
}

and I have a situation that arises like this:

Derived = Base;

Is this possible? What is this operation called? How would I do such a thing?

Thanks for your help.

Answer 1

This is a perfectly ordinary user-defined assignment that looks enough like "slicing", treating a base class as if it were a member, to confuse the unwary. But it's not slicing. For the situation you have, the two classes might as well not be related, and you define the assignment that way.

struct d : b {
     d &operator=(const b &b_) { /*do the work here*/; return *this; }
};

You might be able to use the base class's copy-assignment operator, or you might not:

struct dx : b {
    dx &operator=(const b &b_)
    {
        this->b::operator=(b_);
        // more work
        return *this;
    }
};

but the compiler gives this no special treatment, it's the same as any function call.