A class template can indeed contain virtual or pure virtual functions. This was employed by Andrei Alexandresu in “Modern C++ Design” to implement the visitor pattern using templates and type lists. You can see the code here in his Loki library if you’re interested. With most standard C++ implementations, this is fine, because when the … Read more
The problem is that you cannot mix static time polymorphism (templates) with runtime polymorphism easily. The reason for the language disallowing the particular construct in your example is that there are potentially infinite different types that could be instantiating your template member function, and that in turn means that the compiler would have to generate … Read more
Because base is constructed first and hasn’t “matured” into a derived yet. It can’t call methods on an object when it can’t guarantee that the object is already properly initialized.
Virtual and abstract are almost the same. A virtual method has an implementation in the base class that can optionally be overridden, while an abstract method hasn’t and must be overridden in a child class. Otherwise they are the same. Choosing between them depends on the situation. If you got a base implementation, you use … Read more
The g of the derived class will be called. If you want to call the function in the base, call Base::g(); instead. If you want to call the derived, but still want to have the base version be called, arrange that the derived version of g calls the base version in its first statement: virtual … Read more
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 … Read more
Virtual functions only have a very small performance overhead compared to direct calls. At a low level, you’re basically looking at an array lookup to get a function pointer, and then a call via a function pointer. Modern CPUs can even predict indirect function calls reasonably well in their branch predictors, so they generally won’t … Read more
Virtual inheritance is used to solve the DDD problem (Dreadful Diamond on Derivation). Look at the following example, where you have two classes that inherit from the same base class: class Base { public: virtual void Ambig(); }; class C : public Base { public: //… }; class D : public Base { public: //… … Read more
The Google API includes Google Maps and other Google-specific libraries. The Android one only includes core Android libraries. As for which one to choose, I would go with the Android API until you find that you need the Google API; such as when you need Google Maps functionality. Here is a link to the Google … Read more
The assignment operator is not required to be made virtual. The discussion below is about operator=, but it also applies to any operator overloading that takes in the type in question, and any function that takes in the type in question. The below discussion shows that the virtual keyword does not know about a parameter’s … Read more