The memory layout and the vtable layout depend on your compiler. Using my gcc for instance, they look like this: sizeof(int) == 4 sizeof(A) == 8 sizeof(B) == 8 sizeof(C) == 20 Note that sizeof(int) and the space needed for the vtable pointer can also vary from compiler to compiler and platform to platform. The … Read more
Without virtual tables you wouldn’t be able to make runtime polymorphism work since all references to functions would be bound at compile time. A simple example struct Base { virtual void f() { } }; struct Derived : public Base { virtual void f() { } }; void callF( Base *o ) { o->f(); } … Read more
Why the error & how to resolve it? You need to provide definitions for all virtual functions in class A. Only pure virtual functions are allowed to have no definitions. i.e: In class A both the methods: virtual ~A(); virtual void doWork(); should be defined(should have a body) e.g.: A.cpp void A::doWork() { } A::~A() … Read more
Those are the offset-to-top (needed for multiple inheritence) and typeinfo (RTTI) pointers. From the Itanium ABI (you are not using the Itanium compiler, but their description of this is really good): The offset to top holds the displacement to the top of the object from the location within the object of the virtual table pointer … Read more
It is because the unit generated by MOC isn’t included in the linking process. Or maybe it isn’t generated at all. The first thing I’d do is to put the class declaration in a separate header file, perhaps the build system isn’t scanning implementation files. Another possibility is that the class in question once didn’t … Read more
If Obstacle is an abstract base class, then make sure you declare all its virtual methods “pure virtual”: virtual void Method() = 0; The = 0 tells the compiler that this method must be overridden by a derived class, and might not have its own implementation. If the class contains any non-pure virtual functions, then … Read more
It is not true that vtable pointers in objects are always the most efficient. My compiler for another language used to use in-object pointers for similar reasons but no longer does: instead it uses a separate data structure which maps the object address to the required meta-data: in my system this happens to be shape … Read more
Any time you add a new call to the Q_OBJECT macro, you need to run qmake again. The vtables issue you’re referring to is directly related to that. Just run qmake and you should be good to go assuming there are no other issues in your code.
How are virtual functions implemented at a deep level? From “Virtual Functions in C++”: Whenever a program has a virtual function declared, a v – table is constructed for the class. The v-table consists of addresses to the virtual functions for classes that contain one or more virtual functions. The object of the class containing … Read more