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 << "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(Base,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;
}