没有抽象类的虚函数表？

Question

I am learning about virtual function tables and their representation by analyzing a binary of a simple program written in Visual C++ (with some optimizations on).

A few days ago I asked this question while being stuck on virtual method table content with identical COMDAT folding on.

Now I'm stuck on something else: whenever I analyze a class, I need to find its Virtual Method Table. I can do this by finding either its RTTITypeDescriptor or _s_RTTIClassHierarchyDescriptor , finding a cross reference on it, which should lead me to the _RTTICompleteObjectLocator . When I find a cross reference to the Complete Object Locator, it is written just before the VMT (basically -1st entry of the VMT).

This approach works on some classes (their names start with C in my program). Then there are classes, that are named with I in the beginning and I am able to pair them with other classes starting with C -- for example there is class CClass and it inherits from IClass . These I -classes are probably serving as interfaces to the C -classes and thus they probably only contain abstract methods.

By searching a cross reference to Type Descriptor or Class Hierarchy Descriptor of any of the I -classes I cannot find anything -- there is no Complete Object Locator that would lead me to the VMT of the class (that should be full of references to pure_virtual call if I am correct about the all-abstract methods in the I -classes and if I understand correctly what VMT of abstract class looks like).

Why do the I -classes have no VMT? Did the compiler optimize it out because it would just be full of references to pure_virtual call and manages it in a different way?

Answer 1

These "interfaces" abstract classes probably have need no user written code in any their constructors and destructors (these either have an empty body and no ctor-init-list, or simply are never user defined); let's call these pure interface classes .

[Pure interface class: concept related but not identical to Java interfaces that are (were?) defined as having zero implementation code, in any member function. But be careful with analogies, as Java interfaces inheritance semantic isn't the same as C++ abstract classes inheritance semantic.]

It means that in practice no used object ever has pure interface class type : no expression ever refers to an object with pure interface type. Hence, no vtable is ever needed, so the vtable, which may have been generated during compilation, isn't included in linked code (the linker can see the symbol of the pure interface class vtable isn't used).