If you don't want a class to be copied, for example, you have to define an empty copy constructor and assignment operator yourself and make them private or protected.
Furthermore, the compiler isn't guaranteed to create versions of these classes that do exactly what you want them to do.
So the outer shell of a properly-written assignment operator would look like this: should be a virtual function. Many C programmers are trained to make everything virtual, and in fact, some older frameworks do just that.
In the specific example of the assignment operator, however, it's not a good idea.
If they have public data members (generally a bad idea), they have the same values.
This doesn't necessarily mean that the objects are identical: some purely internal data members (such as caches) might not be copied, or data members pointing to other objects might end up pointing to different objects that are themselves semantically equivalent, rather than pointing to the same objects.
This can be of any type, but the assignment operator that C automatically generates for you (and therefore, the one we're interested in here) is the one where you have the same type of object on both sides of the = sign.
That means the parameter is either an instance of or a reference to an instance of the same class as the object on the left-hand side.
The difference between the copy constructor and assignment operator is that the copy constructor is a constructor a function whose job it is to turn raw storage into an object of a specific class.
An assignment operator, on the other hand, copies state between two existing objects.