memcpy derived class to base class, why still called base class function

Question

I am reading Inside the C++ Object Model. In section 1.3

So, then, why is it that, given

Bear b; 
ZooAnimal za = b; 

// ZooAnimal::rotate() invoked 
za.rotate(); 

the instance of rotate() invoked is the ZooAnimal instance and not that of Bear? Moreover, if memberwise initialization copies the values of one object to another, why is za's vptr not addressing Bear's virtual table?

The answer to the second question is that the compiler intercedes in the initialization and assignment of one class object with another. The compiler must ensure that if an object contains one or more vptrs, those vptr values are not initialized or changed by the source object .

So I wrote the test code below:

#include <stdio.h>
class Base{
public:
    virtual void vfunc() { puts("Base::vfunc()"); }
};
class Derived: public Base
{
public:
    virtual void vfunc() { puts("Derived::vfunc()"); }
};
#include <string.h>

int main()
{
    Derived d;
    Base b_assign = d;
    Base b_memcpy;
    memcpy(&b_memcpy, &d, sizeof(Base));

    b_assign.vfunc();
    b_memcpy.vfunc();

    printf("sizeof Base : %d\n", sizeof(Base));

    Base &b_ref = d;
    b_ref.vfunc();

    printf("b_assign: %x; b_memcpy: %x; b_ref: %x\n", 
        *(int *)&b_assign,
        *(int *)&b_memcpy,
        *(int *)&b_ref);
    return 0;
}

The result

Base::vfunc()
Base::vfunc()
sizeof Base : 4
Derived::vfunc()
b_assign: 80487b4; b_memcpy: 8048780; b_ref: 8048780

My question is why b_memcpy still called Base::vfunc()

Answer 1

What you are doing is illegal in C++ language, meaning that the behavior of your b_memcpy object is undefined. The latter means that any behavior is "correct" and your expectations are completely unfounded. There's not much point in trying to analyze undefined behavior - it is not supposed to follow any logic.

In practice, it is quite possible that your manipulations with memcpy did actually copy Derived's virtual table pointer to b_memcpy object. And your experiments with b_ref confirm that. However, when a virtual method is called though an immediate object (as is the case with b_memcpy.vfunc() call) most implementations optimize away the access to the virtual table and perform a direct (non-virtual) call to the target function. Formal rules of the language state that no legal action can ever make b_memcpy.vfunc() call to dispatch to anything other than Base::vfunc(), which is why the compiler can safely replace this call with a direct call to Base::vfunc(). This is why any virtual table manipulations will normally have no effect on b_memcpy.vfunc() call.