关于c ++：继承和多态的低级细节

Low level details of inheritance and polymorphism

这个问题是困扰我的一大疑问，也很难用语言来描述它。有时它看起来很明显，有时很难破解。所以问题是这样的：

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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 &lt&lt"Derived from Base" &lt&lt endl;
virtual void function4() {cout &lt&lt"Only in derived" &lt&lt 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指向派生对象，但它访问哪个vtable以及如何访问？因为b eu ptr->function4()给出了编译错误。或者b_ptr只能访问派生vtable中的基类vtable的大小？

Q2。由于派生类的内存布局必须是(base，derived)，那么派生类的内存布局中是否也包括基类的vtable？

Q3。因为基类vtable的function1和function2指向基类实现，派生类的function2指向基类的function2，所以基类中真的需要vtable吗？？(这可能是我问过的最愚蠢的问题，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。)

请评论。

谢谢你的耐心。

相关讨论

作为进一步的说明，这里是C++程序的C版本，显示了VTABLE和ALL。

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#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
");
}

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

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;
}