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
1. Do I have any errors in the above description? All good. 🙂 2. How does the compiler know f’s position in vtable Each vendor will have their own way of doing this, but I always think of the vtable as map of the member function signature to memory offset. So the compiler just maintains … Read more
As others have noted, it is certainly possible to implement this in C. Not only is it possible, it is a fairly common mechanism. The most commonly used example is probably the file descriptor interface in UNIX. A read() call on a file descriptor will dispatch to a read function specific to the device or … Read more
The “virtual function table” or “virtual method table” is a list of method pointers that each class has. It contains pointers to the virtual methods in the class. Each instance of a class has a pointer to the table, which is used when you call a virtual method from the instance. This is because a … Read more
Currently there is no standard way of doing this in C++ although the information must be available somewhere. Otherwise, how could the program call the function? However, GCC provides an extension that allows us to retrieve the address of a virtual function: void (A::*mfp)() = &A::func; printf(“address: %p”, (void*)(b->*mfp)); …assuming the member function has 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
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