How can we make a class abstract?

Answer 1

Add a pure virtual method:

class Shape {
public:
virtual void Draw() = 0;
}

Answer 2

Create a class with all pure virtual methods.

Answer 3

A class which has AT LEAST one pure virtual function is considered abstract.
It can have all types of members and functions (non-pure-virtual too) except that one function must be pure virtual (so that it satisfies above condition).

Answer 4

Abstracr class with all pure virtual methods named interface.