Question

A little question about creating objects. Say I have these two classes:

struct A{
    A(){cout << "A() C-tor" << endl;}
    ~A(){cout << "~A() D-tor" << endl;}
};

struct B : public A{
    B(){cout << "B() C-tor" << endl;}
    ~B(){cout << "~B() D-tor" << endl;}

    A a;
};

and in main I create an instance of B:

int main(){
    B b;
}

Note that B derives from A and also has a field of type A.

I am trying to figure out the rules. I know that when constructing an object first calls its parent constructor, and vice versa when destructing.

What about fields (A a; in this case)? When B is created, when will it call A‘s constructor? I haven’t defined an initialization list, is there some kind of a default list? And if there’s no default list? And the same question about destructing.

Answer 1

• Construction always starts with the base class. If there are multiple base classes then, construction starts with the left most base. (side note: If there is a virtual inheritance then it’s given higher preference).
• Then the member fields are constructed. They are initialized in the
  order they are declared
• Finally, the class itself is constructed
• The order of the destructor is exactly the reverse

Irrespective of the initializer list, the call order will be like this:

1. Base class A‘s constructor
2. class B‘s field named a (of type class A) will be constructed
3. Derived class B‘s constructor