Question

#include<iostream>
using namespace std;

class A
{
public:
        int i;
        A() {cout<<"A()"<<endl;}
        ~A() {cout<<"~A()"<<endl;}
};
class B:public A
{
public:
        int j;
        B(): j(10)
        {
                this->i=20;
                this->~A();
        }
};

int main()
{
        B abc;
        cout<<"i="<<abc.i<<" j="<<abc.j<<endl;
}//main

Two questions:

1. How come A’s destructor gets called like an ordinary function instead of destroying the object? (or is it some kind of rule that the base class will be destroyed only if the child class’s destructor calls the base class’s destructor?) I was trying out this sample code to find out how the destructor works. So if simply calling the destructor function does not destruct the object, then there is obviously some other kind of call that calls the destructor and only then the object is destructed. What’s so special in that kind of call and what call is it?

2. Is there a way to have an initialization list for A in B’s constructor? Something like this:

   class B:public A
   { 
       B(): j(10), A():i(20) {}
   };
   

Answer 1

@Nav: no, your understanding of “destroyed” is just wrong. When an object’s destructor is called, the object is destroyed. You seem to believe that the memory it resided in evaporates entirely, but that never happens. The object no longer exists, but some garbage data is typically left over by the object, and if you’re willing to break the rules of C++ and invoke undefined behavior, then you can read those leftover bytes, and they’ll look like the object, and because there are no runtime checks on whether you’re accessing a valid object, you can often treat them as an object. Which you do.

It’s illegal, it’s undefined behavior, but in practice it often works.

Once again, a destructor does not physically vaporize the memory. Your RAM still has the same capacity after a destructor has executed. Conceptually, the object no longer exists once the destructor has run. But the data it contained is still there in memory.