## 21.4 Pure Virtual Member Functions

- abstract class: contains a pure virtual method
- Provides an i/f, but no behavior
- Cannot be instantiated
- Used only as a base class for inheritance
- E.g., compute the area of a shape:
  - `Shape` must declare an `area` method
  - Can't define it
  - Each derived class has an `area` method
- interface - class where all methods are pure virtual

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