Question

A derived class's constructor explicitly invokes its base class's constructor, but a derived class's destructor cannot invoke its base class's destructor. Why does this apparent asymmetry make sense?

Short answer

Derived class's constructor must call the base class's constructor for proper initialization; destructors naturally run in reverse order.

Step-by-step solution

Understanding Inheritance

Inheritance is a feature in object-oriented programming where a class (derived class) inherits properties and behaviors from another class (base class). The derived class can add new properties or methods or override existing ones.

Constructors in Derived Classes

When a derived class is instantiated, it often needs to initialize components inherited from the base class. This is why a derived class's constructor explicitly invokes the base class's constructor using a special syntax (e.g., `BaseClassConstructor()` in C++ or `super()` in Java). This ensures that the base class's initialization code runs first.

Destructors in Derived Classes

When an object of a derived class is destroyed, destructors are called in the reverse order of constructors. The derived class’s destructor runs first, followed by the base class’s destructor. Therefore, the derived class does not explicitly call the base class's destructor.

Why the Asymmetry Makes Sense

Constructors and destructors serve different purposes: constructors build and initialize objects, while destructors clean up resources. Explicitly calling the base class's constructor is necessary for proper initialization, but explicit calls to destructors are not needed due to the natural order of destruction.

Key concepts

Constructors in Derived Classes

When creating a derived class in object-oriented programming (OOP), the derived class often relies on foundational elements established in its base class. Hence, it is important to initialize these base class components first. This initialization is achieved through the derived class's constructor.
In languages like C++, this is done by explicitly invoking the base class constructor in the initialization list. For example, if `BaseClass` has a constructor `BaseClass(int param)`, you might see a derived class initialization such as:

`DerivedClass(int param1, int param2) : BaseClass(param1), derivedParam(param2) { }`.

In Java, the `super()` function is used. If `BaseClass` takes parameters in its constructor, calling `super(param1)` within the derived class constructor ensures the base class components are correctly set up first. This super call is required for proper initialization, which ensures the derived class has a solid foundation on which to build its additional properties and behaviors.

Destructors in Derived Classes

In OOP, destructors are special methods called when an object is destroyed. Their primary job is to release resources that the object may have acquired during its lifetime. In contrast to constructors, destructors follow a natural reverse order of invocation.
When an object of a derived class is destroyed, the destructor of the derived class is called first. Once the derived class has cleaned up its resources, the base class destructor is automatically called to handle the cleanup of base class resources. This reverse order ensures that dependent resources are cleaned up correctly and in a controlled manner.

Thus, there is no need for the derived class's destructor to explicitly call the base class's destructor. The programming language runtime system handles this ordering. For instance, if you have `~DerivedClass()`, you do not need to call `~BaseClass()` inside it.

Base Class Initialization

Initialization of a base class is a critical step in constructing an object of a derived class. This step ensures that all inherited properties are set up before the derived class adds its specific attributes.
When a derived class constructor is invoked, it must first call the base class constructor. This guarantees that all the necessary groundwork laid by the base class is in place.
Consider an example where `BaseClass` has a constructor that initializes some core properties like
`BaseClass(int coreProperty)`. Any derived class must call this constructor to ensure `coreProperty` is correctly set.
As highlighted earlier, this might look like `DerivedClass(int coreProperty, int additionalProperty) : BaseClass(coreProperty)`.
Without this step, the derived class would be left in an uninitialized or half-initialized state, likely leading to errors or undefined behaviors.

In summary, explicit invocation of the base class constructor by the derived class ensures coherent and reliable initialization of objects.