继承和多态性的低级细节

这个问题是我心中的一大疑惑，也很难用语言来形容。有时它看起来很明显，有时却很难破解。所以问题是这样的：：

class Base{
public:
     int a_number;

     Base(){}
     virtual void function1() {}
     virtual void function2() {}
     void function3() {}
};

class Derived:public Base{
public:
     Derived():Base() {}
     void function1() {cout << "Derived from Base" << endl; 
     virtual void function4() {cout << "Only in derived" << endl;}
};

int main(){

      Derived *der_ptr = new Derived();
      Base *b_ptr = der_ptr;  // As just address is being passed , b_ptr points to derived                object

      b_ptr -> function4(); // Will Give Compilation ERROR!!

      b_ptr -> function1(); // Calls the Derived class overridden method

      return 0;

}
  

Q1.虽然 b_ptr 指向 Derived 对象，但它访问哪个 VTABLE 以及如何访问？因为 b_ptr -> function4() 给出编译错误。或者 b_ptr 只能访问派生 VTABLE 中基类 VTABLE 的大小？

Q2。既然 Derived 的内存布局一定是 (Base,Derived) ，那么 Base 类的 VTABLE 是否也包含在 Derived 类的内存布局中呢？

Q3。既然基类Vtable的function1和function2指向Base类的实现，而派生类的function2指向Base类的function2，那么基类中真的需要VTABLE吗？ （这可能是我能问的最愚蠢的问题，但在我目前的状态下我仍然对此表示怀疑，答案必须与问题1的答案相关:)）

请给出意见。

感谢您的耐心等待。

作为进一步的说明，这里是 C++ 程序的 C 版本，显示了 vtable 等。

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

typedef struct Base Base;
struct Base_vtable_layout{
    void (*function1)(Base*);
    void (*function2)(Base*);
};

struct Base{
    struct Base_vtable_layout* vtable_ptr;
    int a_number;
};

void Base_function1(Base* this){}

void Base_function2(Base* this){}

void Base_function3(Base* this){}

struct Base_vtable_layout Base_vtable = {
    &Base_function1,
    &Base_function2
};

void Base_Base(Base* this){
    this->vtable_ptr = &Base_vtable;
};

Base* new_Base(){
    Base *res = (Base*)malloc(sizeof(Base));
    Base_Base(res);
    return res;
}

typedef struct Derived Derived;
struct Derived_vtable_layout{
    struct Base_vtable_layout base;
    void (*function4)(Derived*);
};

struct Derived{
    struct Base base;
};

void Derived_function1(Base* _this){
    Derived *this = (Derived*)_this;
    printf("Derived from Base\n");
}

void Derived_function4(Derived* this){
    printf("Only in derived\n");
}

struct Derived_vtable_layout Derived_vtable = 
{
    { &Derived_function1,
      &Base_function2},
    &Derived_function4
};

void Derived_Derived(Derived* this)
{
    Base_Base((Base*)this);
    this->base.vtable_ptr = (struct Base_vtable_layout*)&Derived_vtable;
}      

Derived* new_Derived(){
    Derived *res = (Derived*)malloc(sizeof(Derived));
    Derived_Derived(res);
    return res;
}



int main(){
      Derived *der_ptr = new_Derived();
      Base *b_ptr = &der_ptr->base;
      /* b_ptr->vtable_ptr->function4(b_ptr); Will Give Compilation ERROR!! */
      b_ptr->vtable_ptr->function1(b_ptr);
      return 0;
}