How are Constructors and Destructors Called in C Inheritance?

Order of Constructor and Destructor Calls in Inheritance

In object-oriented programming with inheritance, understanding the order of constructor and destructor calls is crucial. This becomes particularly important when dealing with multiple base classes and object compositions.

Consider the following class hierarchy:

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;
};

In the main function:

int main() {
    B b;
}

Here are the rules governing the order of constructor and destructor calls in this inheritance hierarchy:

Constructor Calls:

1. Base Class Construction: Constructor calls begin with the base class. In this case, A's constructor is called first.
2. Member Field Construction: Next, member fields are constructed in the order they are declared in the derived class. In this case, B's field a of type A is constructed.
3. Derived Class Construction: Finally, the constructor of the derived class, B, is called.

Destructor Calls:

The order of destructor calls is the reverse of the constructor call order:

1. Derived Class Destructor: The destructor of the derived class, B, is called first.
2. Member Field Destructor: Next, the destructor of the member field a is called, destroying its instance of A.
3. Base Class Destructor: Finally, the destructor of the base class, A, is called.

Default Initialization List:

Even without an explicitly defined initialization list, the member field will be initialized before the derived class constructor is called. In this case, a would be initialized to the default constructor of A.

Therefore, the expected output for the code in the main function would be:

A() C-tor
A() C-tor
B() C-tor
~B() D-tor
~A() D-tor
~A() D-tor

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