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
This is of course implementation-dependent. And it would make a terrible interview question. A good C++ programmer can just trust sizeof to be right and let the compiler worry about those vtable things. But what’s going on here is that a typical vtable-based implementation needs two vtables in objects of class C or D. Each … Read more
Depends on compiler. In VC++, the vtable pointer stored at the beginning of the object allocation, before any member data. (Provided your class has at least one virtual member function.) There also may be multiple vtable pointers, if your class multiply-inherits from other classes with vtables. The vtables themselves are statically allocated somewhere in your … Read more
Syntax errors non-withstanding, you cannot dynamic_cast a non-polymorphic type. static_cast is the cast you would use in this case, if you know that it is in fact an object of the target type. The reason why: static_cast basically has the compiler perform a check at compile time “Could the input be cast to the output?” … 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
V-tables (or virtual tables) are how most C++ implementations do polymorphism. For each concrete implementation of a class, there is a table of function pointers to all the virtual methods. A pointer to this table (called the virtual table) exists as a data member in all the objects. When one calls a virtual method, we … 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