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## Calling Base-Class Member Functions

- Suppose `B``::f` is a function in the base class. The the derived class `D` can take three kinds of actions.
  - The derived class can extend `B``::f` by supplying a new implementation that refers to the base-class implementation. For example, the `TravelClock::get_hours` function is an extension of `Clock::get_hours`.
  - The derived class can replace `B``::f` by supplying a new implementation that is unrelated to the base-class implementation. For example, `TravelClock::get_location` is a replacement for `Clock::get_location`.
  - The derived class can inherit `B``::f` by not supplying an implementation for `f`. For example, `TravelClock` inherits `Clock::get_minutes` and `Clock::is_military`.

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