why not have a virtual ptr per class?-CodePudding

It seems that it's sufficient to have a virtual ptr per class even when a class is derived from two base classes.

For example,

#include <iostream>
class A{
public:
    void* ptr;
    virtual void printNonOverride() {
        std::cout << "no override" << std::endl;
    }
    virtual void print() {
        std::cout << "hello world!" << std::endl;
    }
};

class C {
public:
    double c;
    virtual void cFunc() {
        std::cout << "C";
    }
};

class B : public A , public C {
};

int main() {
    A a{};
    std::cout << sizeof a << std::endl;
    C c{};
    std::cout << sizeof c << std::endl;
    B b{};
    std::cout << sizeof b << std::endl;
}

The output for this is

16
16
32

which makes sense because sizeof B = sizeof A sizeof C. However, this seems a little bit inefficient. Why does B need to allocate 2 virtual pointers? Why not just allocate one that refers to B's own virtual tables? Then the sizeof B will be 24. Thank you!

CodePudding user response：

Because you can do

B b{};
C* pc = &b;

pc->cFunc();

and pc has to point to something that looks like a C - including starting with a virtual pointer suitable for a C.

B and A can share the same virtual pointer, because the compiler can make it so that B's virtual table begins the same way as A's virtual table (and then has some extra stuff at the end). But it can't do this for both A and C at the same time. B's virtual table can't begin the same way as A's and begin the same way as C's. That's not possible.