non-virtual and virtual functions

/:#> cat t.C

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>

 

class A {

public:
    A():a(1) {}
public:
    int a;
};

class B : public A {
public:
    B():b(2) {}
    virtual ~B() {}
public:
    int b;
};


int main(int argc, char * argv[])
{
    B *o = new B();

    char * p = (char *)o;

    A *a = (A*)p;
    B *b = (B*)p; // correct
    //B *b = (B*)a; // incorrect

    A *c = (A *)b;

    printf("o/t=0x%x/n",o);
    printf("p/t=0x%x/n",p);
    printf("a/t=0x%x,a.a=%d/n",a,a->a);
    printf("b/t=0x%x,b.b=%d/n",b,b->b);
    printf("c/t=0x%x,c.a=%d/n",c,c->a);

}

 

 

class A 没有虚函数, 而class B有虚函数。

所以class B的头字节 和 class A的头字节是差一个虚函数表指针偏移, &A = &B + 4 (假定字长是4)。