Are abstract methods and pure virtual functions the same thing?

c++ function virtual
Ahmad picture Ahmad · Jul 4, 2011 · Viewed 47.8k times · Source

As far as I know, both abstract methods and pure virtual functions do NOT provide any functionality ... So can we say they're both the same thing ?

Also, suppose a class (not necessarily declared as abstract) contains a number of implemented methods (not abstract or virtual), but contains a pure virtual function. Is this class then abstract ?

Answer

Gordon Gustafson picture Gordon Gustafson · Jul 4, 2011

Yes, they are the same thing. In C++, an abstract method is just another way of describing the characteristics of a pure virtual function. Both just mean a method with no implementation provided that needs to be implemented in a sub-class before the class can actually be instantiated.

The situation with pure virtual functions and abstract classes in C++ is similar as they essentially mean exactly the same thing. Any abstract class must have at least 1 pure virtual function or else it could be instantiated and wouldn't be abstract. Likewise, any class with at least 1 pure virtual function must be abstract because it needs to be extended so that method can actually be implemented.

Therefore, a class is abstract if and only if it contains at least 1 pure virtual function/abstract method.

Later on, languages like Java and C# made things like this more explicit, allowing a special keyword to define a class abstract rather than the presence of a pure-virtual function. C++ lets you do the same things as these languages, but they're just a little more explicit about it. :D

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