c – 继承和多态的低级细节

概述这个问题是我头脑中隐藏的一大疑问,也难以用言语来描述.有时似乎很明显,有时候很难破解.所以问题就像这样:: class Base{public: int a_number; Base(){} virtual void function1() {} virtual void function2() {} void function3() {} 这个问题是我头脑中隐藏的一大疑问,也难以用言语来描述.有时似乎很明显,有时候很难破解.所以问题就像这样::

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Derived对象,但它访问的Vtable和HOW？ as b_ptr – > function4()给出了编译错误.或者是b_ptr只能访问Derived Vtable中基本类Vtable的大小？

Q2.由于Derived的内存布局必须是(B​​ase,Derived),Base类的Vtable是否也包含在Derived类的内存布局中？

Q3.由于基类Vtable的function1和function2指向基类实现,Derived类的function2指向Base类的function2,在Base类中是否真的需要Vtable？ (这可能是我可以提出的最愚蠢的问题,但我仍然对目前的状态存在疑问,答案必须与Q1的答案相关:))

请评论.

谢谢你的耐心等待.

解决方法 作为进一步说明,这是您的C程序的C版本,显示vtable和所有.

#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;}

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