Providing override key work allows for extra checking
that the function signatures are correct.

https://stackoverflow.com/questions/39932391/virtual-override-or-both-c?answertab=active#tab-top

In the derived class the function is virtual by way of having the ¹same
type as the base class function.

However, an override can help avoid bugs by producing a compilation
error when the intended override isn't technically an override. E.g.
that the function type isn't exactly like the base class function. Or
that a maintenance of the base class changes that function's type, e.g.
adding a defaulted argument.

In the same way, a virtual keyword in the derived class can make such a
bug more subtle, by ensuring that the function is still virtual in
further derived classes.

The general advice is,
     -Use virtual for the base class function declaration.
      This is technically necessary.

     -Use override (only) for a derived class' override.
      This helps maintenance.