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by parenthephobia
3741 days ago
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C++ doesn't have the interface keyword, but it also doesn't have interfaces. An interface isn't simply a class whose methods are all abstract. It is a distinct kind of type which has no precise analogue in C++ because the language constraints which enable it to exist don't apply in C++. Namely, C++ has multiple inheritance. To abuse a car analogy, this situation is akin to asking why older drivers weren't taught to use cruise control, and the answer being that many older cars don't have cruise control. Whilst it's possible to "emulate" cruise control by simply maintaining constant speed manually, learning to do that isn't the same as learning how to use cruise control. |
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From [1]:
"Bjarne Stroustrup: I had a lot of problems explaining that to people and never quite understood why it was hard to understand. From the first days of C++, there were classes with data and classes without data. The emphasis in the old days was building up from a root with stuff in it, but there were always abstract base classes. In the mid to late eighties, they were commonly called ABCs (Abstract Base Classes): classes that consisted only of virtual functions. In 1987, I supported pure interfaces directly in C++ by saying a class is abstract if it has a pure virtual function, which is a function that must be overridden. Since then I have consistently pointed out that one of the major ways of writing classes in C++ is without any state, that is, just an interface.
From a C++ view there's no difference between an abstract class and an interface. Sometimes we use the phrase "pure abstract class," meaning a class that exclusively has pure virtual functions (and no data). It is the most common kind of abstract class. When I tried to explain this I found I couldn't effectively get the idea across until I introduced direct language support in the form of pure virtual functions. Since people could put data in the base classes, they sort of felt obliged to do so. People built the classic brittle base classes and got the classic brittle base class problems, and I couldn't understand why people were doing it. When I tried to teach the idea with abstract base classes directly supported in C++, I had more luck, but many people still didn't get it. I think it was a major failure in education on my part. I didn't imagine the problem well. That actually matches some of the early failures of the Simula community to get crucial new ideas across. Some new ideas are hard to get across, and part of the problem is a lot of people don't want to learn something genuinly new. They think they know the answer. And once we think we know the answer, it's very hard to learn something new. Abstract classes were described, with several examples, in The C++ Programming Language, Second Edition, in 1991, but unfortunately not used systematically throughout the book. "
[1] http://www.artima.com/intv/modern.html