Is virtual table necessary for C++?

Question

I have a doubt about C++ virtual table recently.

Why does C++ use virtual table?

=>Because C++ compiler does not know the actual function address

--->Why?

=>Because C++ compiler does not know the exact type(Cat? Dog? Animal?) of the object the pointer "panimal" points to

---Why? Is that any way compiler can figure out the object type?

=>Yes, I think the compiler can make it via tracking object type.

Let's consider the sources where an object pointer gets its value. 2 sources indeed.

1. another pointer
2. address of class instance Where does "another pointer" get its value? Eventually, there's a pointer that gets its value from "class instance".

So, via tracking the assignment thread backwards to the original source object

　　=> the compiler is able to figure out the exact type of a pointer.

　　=>the compiler knows the address of the exact function being called

　　=>no virtual table is needed.

Object type tracking saves both virtual table memery and virtual table pointer of each class instances.

Where does object type tracking not work?

Library Linking.

If a library function returns a base-class pointer, there's no way for the compiler to track back to the original source object. The compiler can probably adapt to library code and none-library code. For library classes that are exported out, use virtual table. For other classes, just track theire object type to save memory.

I am not sure whether there's some error in above statements, please kindly point it out if any. Thanks in advance~

Answer 1

In some cases, yes, the compiler can figure out the type a pointer points to at compile time. It is quite easy to construct a case where it cannot though.

int x;
cin >> x;
Animal* p;
if (x == 10)
    p = new Cat();
else
    p = new Dog();

If the compiler can, in all cases, prove the type of an object, it is free to eliminate virtual tables from its generated code, as per the as-if rule.