So "virtual void attack() = 0" = "virtual void attack(){}"?

Answer 1

A big NO.
virtual void attack() =0; is a pure virtual function. It can not have a definition.
virtual void attack(){} is not pure virtual function. it may have definition.

Answer 2

no. virtual mean that derived class must implement our virtual function. base class have only info that our function have specific type and name , no implementation.