Question

#include <iostream>
#include <string>

using namespace std;

class Person
{
    string name;

    public:
        Person():name("")
        {
            cout << "Person default ctor\n";
        }
        Person(const string& name_in):name(name_in)
        {
            cout << "Person string ctor: " << name << "\n";
        }
        ~Person()
        {
            cout << "Person dtor: " << name << "\n";
        }
        string get_name()
        {
            return name;
        }
};

class Professor:public Person
{
    int office;

    public:
        Professor(const string& name_in, int office_in):Person(name_in), office(office_in)
        {
            cout << "Professor string ctor: " << get_name() << endl;
        }
        ~Professor()
        {
            cout << "Professor dtro: " << get_name() << endl;
        }
};

int main()
{
    Person alice("Alice");
    Professor bob("Bob", 10);

    return 0;
}

I would assume that the output should be:

Person string ctor: Alice

Person dtor: Alice

Professor string ctor: Bob

Professor dtor: Bob

Since it seems like that should logically follow from the structure of the program. However, the real output is:

Person string ctor: Alice

Person string ctor: Bob

Professor string ctor: Bob

Professor dtor: Bob

Person dtor: Bob

Person dtor: Alice

Can someone explain why this is? What am I not understanding about classes/constructors/destructors that’s making me come up with the wrong output?

Answer 1

First of all, Professor derives from Person. This means that every instance of Professor implicitly contains an instance of Person (observe how you’re calling Person(name_in) in Professor‘s constructor). This implicit instance will automatically get destroyed when the Professor object is being destroyed.

This explains why you see:

Person string ctor: Bob
Person dtor: Bob

As to the ordering of the destructor calls, the variables are destroyed in the reverse order of construction. This explains why Bob is destroyed before Alice:

Person string ctor: Alice
Professor string ctor: Bob
Professor dtor: Bob
Person dtor: Alice