C++ Destructor inheritance

Question

/* Problem 50 */
#include <iostream>
using namespace std;

class a {
    char ach;
  public:
    a(char c) { ach = c - 1; }
    ~a(); // defined below
    virtual void out(ostream &os) {
      if ('m' < ach)
        os << ach << char(ach+7) << char(ach+6) << ' ';
      else
        os << ach << ach << ach;
    }
};

class b: public a {
    char bach;
  public:
    b(char c1, char c2) : a(c1) { bach = c2-1; }
    void out(ostream &os) {
      a::out(os);
      os << ' ' << bach << char(bach + 11);
    }
};

ostream &operator<<(ostream &os, a &x) {
  x.out(os);
  return os;
}

a::~a() {
  cout << *this; // calls above operator
}

int main() {
  b var1('n', 'e');
  a var2('o');
  cout << "Homer says: " << var1 << '\n';
  return 0;
}

I'm confuse why only two object being destruct while there are three object being construct

I also have put cout on each of the construct on the base_class and the derived_class to see how many were construct and I was right about the number of constructed object, but I was wrong when I did the destruct.

If anyone could please point me out why the last destruct didn't apply to the first object that being create?

Answer 1

There are only two objects being constructed, the 3 couts you see in the constructor is because of the fact that when you create a derived class object the base class constructor is called. As a side note, you need to declare class a destructor as virtual.

Answer 2

when you are creating the first object in the main b var1('n', 'e'); which means that this object will be constructed by class b constructor b(char c1, char c2) : a(c1) and you are telling it to also use class a constructor which means that you have already have called 2 constructors in this case. and the last constructor is for this object a var2('o'); in this case you are using the constructor in class a. so in total you have used 2 constructors in class a and 1 constructor in class b. you have 2 objects and the reason why you see 2 objects being destructed is because you have a ~a() but you don't have ~b().

hope this will help