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Discussion
Despite two decades of object-oriented design knowledge, the purpose and practice of public inheritance are still frequently misunderstood, and many uses of inheritance are flawed.Public inheritance must always model "is-a" ("works-like-a") according to the Liskov Substitution Principle (see [Liskov88]): All base contracts must be fulfilled, and so all overrides of virtual member functions must require no more and promise no less than their base versions if they are to successfully fulfill the base's contract. Code using a pointer or reference to a Base must behave correctly even when that pointer or reference actually points to a Derived .Item 39)Liskov88]). The purpose of public inheritance is not for the derived class to reuse base class code to implement itself in terms of the base class's code. Such an is-implemented-in-terms-of relationship can be entirely proper, but should be modeled by compositionor, in special cases only, by nonpublic inheritance (see Item 34).Item 36)Before object orientation, it has always been easy for new code to call existing code. Public inheritance specifically makes it easier for existing code to seamlessly and safely call new code. (So do templates, which provide static polymorphism that can blend well with dynamic polymorphism; see Item 64)
