If you have an interface hierarchy and a corresponding implementation hierarchy, making the interface base classes virtual bases is necessary. E.g. struct IBasicInterface { virtual ~IBasicInterface() {} virtual void f() = 0; }; struct IExtendedInterface : virtual IBasicInterface { virtual ~IExtendedInterface() {} virtual void g() = 0; }; // One possible implementation strategy struct CBasicImpl … Read more
The page “Notes on Multiple Inheritance in GCC C++ Compiler v4.0.1” is offline now, and http://web.archive.org didn’t archive it. So, I have found a copy of the text at tinydrblog which is archived at the web archive. There is full text of the original Notes, published online as part of “Doctoral Programming Language Seminar: GCC … Read more
Because it avoids this: class A { public: A(int) {} }; class B0: virtual public A { public: B0(): A(0) {} }; class B1: virtual public A { public: B1(): A(1) {} }; class C: public B0, public B1 { public: C() {} // How is A constructed? A(0) from B0 or A(1) from B1? … Read more
Virtual bases are very different from ordinary bases. Remember that “virtual” means “determined at runtime” — thus the entire base subobject must be determined at runtime. Imagine that you are getting a B & x reference, and you are tasked to find the A::a member. If the inheritance were real, then B has a superclass … Read more
The constructors for virtual base classes are always called from the most derived class, using any arguments it might pass in. In your case, the most derived class doesn’t specify an initializer for A, so the default constructor is used.
[*] The technical problem is that there’s no way to work out from a Base* what the offset is between the start of the Base sub-object and the start of the Derived object. In your example it appears OK, because there’s only one class in sight with a Base base, and so it appears irrelevant … Read more
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
When using virtual inheritance, the virtual base class’s constructor is called directly by the most derived class’s constructor. In this case, the daughter constructor directly calls the grandmother constructor. Since you didn’t explicitly call grandmother constructor in the initialization list, the default constructor will be called. To call the correct constructor, change it to: daugther(int … Read more
You want: (Achievable with virtual inheritance) A / \ B C \ / D And not: (What happens without virtual inheritance) A A | | B C \ / D Virtual inheritance means that there will be only 1 instance of the base A class not 2. Your type D would have 2 vtable pointers … Read more
virtual base classes are special in that they are initialized by the most derived class and not by any intermediate base classes that inherits from the virtual base. Which of the potential multiple initializers would the correct choice for initializing the one base? If the most derived class being constructed does not list it in … Read more