How does inheritance promote code reuse?

Answer 1

One of the earliest motivations for using inheritance was the re-use of code which already existed in another class. This practice is usually called implementation inheritance.

In most quarters, class inheritance for the sole purpose of code re-use has fallen out of favor.The primary concern is that implementation inheritance does not provide any assurance of polymorphic substitutability-an instance of the re-using class cannot necessarily be substituted for an instance of the inherited class. An alternative technique, delegation, requires more programming effort but avoids the substitutability issue. In C++ private inheritance can be used as form of implementation inheritance without substitutability. Whereas public inheritance represents an "is-a" relationship and delegation represents a "has-a" relationship, private (and protected) inheritance can be thought of as an "is implemented in terms of" relationship.