What's the Exact Call Order of Constructors and Destructors in Inheritance?

Understanding Constructor and Destructor Call Order in Inheritance

In object-oriented programming, classes can inherit from base classes, creating a hierarchical relationship where the derived class inherits the properties and behaviors of the parent class. When creating and destroying objects of these classes, the order of constructor and destructor calls plays a crucial role in initializing and finalizing the objects.

For the given example:

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;  // Field of type A in class B
};

And the following code in main:

int main() {
    B b;
}

Construction Order

1. Base Class Construction: The construction process always begins with the base class constructor. In this case, A is the base class, so its constructor A() is called first.
2. Member Field Construction: After the base class is constructed, the member fields are initialized in the order they are declared. Here, B has a field a of type A. Since there is no initializer list explicitly defined, a default initializer is used, which simply calls the constructor for a.
3. Derived Class Construction: Once the member fields are initialized, the derived class constructor can be called. In this case, B() is called.

Destruction Order

The order of destructor calls is exactly the reverse of the construction order:

1. Derived Class Destruction: First, the derived class destructor ~B() is called.
2. Member Field Destruction: Next, the member fields are destructed in reverse order of their declaration. The field a of type A will be destructed.
3. Base Class Destruction: Finally, the base class destructor ~A() is called.

Irrespective of whether an initializer list is used or not, the call order for construction and destruction will follow these principles, ensuring proper initialization and cleanup of objects in the inheritance hierarchy.

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