Question

#include <cstdio>
#include <cstring>

class A
{
public:
    virtual void foo()
    {
        puts("A");
    }
};

class B : public A
{
public:
    void foo()
    {
        puts("B");
    }
};

int main()
{
    A a;
    B b;
    memcpy(&a, &b, sizeof (b));
    (&a)->foo();
}

Answer 1

You are not supposed to mess with non-POD types like that. In particular, the C++ standard says that memcpying non-PODs results in undefined behavior, which, in your case, shows as continuing to see a as being of type A.

In your particular case, the compiler “knows” that the both the “static type” and the “dynamic type” of a is A (since its type can’t “legally” change – your trick is illegal), so no virtual dispatch is performed, but a.foo() is called directly (and your trick of overwriting the vptr thus has no effect).