11/13/2023 0 Comments Inheritance 2020 cast![]() In our degenerate case, Person::speak() is not overridden in either Student or Worker, but that could be different and then we would TeachingAssistant would have multiple implementations of the speak() method. The real-world meaning of “exists only once” is that a TeachingAssistant should have only one way of implementing speak, not two different ways. The fact that a TeachingAssistant is a Student and is a Worker at the same time does not imply that a TeachingAssistant is a Person twice (unless the TA suffers from schizophrenia): a Person base class corresponds to a contract that TeachingAssistant implements (the “is a” relationship above really means “implements the requirements of”), and a TeachingAssistant only implements the Person contract once. In this case, the double inheritance of Person is probably unwanted, as we want to model that the relation between TeachingAssistant and a Person exists only once. Explicit qualification not only uses an easier, uniform syntax for both pointers and objects but also allows for static dispatch, so it would arguably be the preferable way to do it. In order to call speak(), the same disambiguation, or explicit qualification is needed: static_cast(ta).speak() or static_cast(ta).speak() or alternatively ta.Student::speak() and ta.Worker::speak(). TeachingAssistant ta Person & student = static_cast ( ta ) Person & worker = static_cast ( ta ) This means the classes above the “join class” (the one in the bottom) have very little if any data.Ĭonsider the following class hierarchy to represent the diamond problem, though not with pure abstracts. In practice, virtual base classes are most suitable when the classes that derive from the virtual base, and especially the virtual base itself, are pure abstract classes. ![]() Instead, if classes B and C inherit virtually from class A, then objects of class D will contain only one set of the member variables from class A.Īs you probably guessed, this technique is useful when you have to deal with multiple inheritance and it’s a way to solve the infamous diamond inheritance. These will be accessible independently, using scope resolution. Without virtual inheritance, if two classes B and C inherit from class A, and class D inherits from both B and C, then D will contain two copies of A’s member variables: one via B, and one via C. Virtual inheritance is a C++ technique that ensures that only one copy of a base class’s member variables are inherited by second-level derivatives (a.k.a. What is virtual inheritance? The diamond problem In C++, this might mean private inheritance or virtual inheritance. At those times, we might learn about some more specialized forms of inheritance. But we still need inheritance, and from time to time we run into problems where it seems to be the only way. ![]() A bit later we learn that we should rather use composition over inheritance. When we start coding in an object-oriented programming language we often think that it’s about building nice inheritance hierarchies.
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