A pure virtual function is usually not (but can be) implemented in a base class and must be implemented in a leaf subclass.
You denote that fact by appending the "= 0" to the declaration, like this:
class AbstractBase
{
virtual void PureVirtualFunction() = 0;
}
Then you cannot declare and instantiate a subclass without it implementing the pure virtual function:
class Derived : public AbstractBase
{
virtual void PureVirtualFunction() override { }
}
By adding the override keyword, the compiler will ensure that there is a base class virtual function with the same signature.