Kerala Plus Two Computer Science (DHSE): Classes, Objects, Constructors and Inheritance

Classes, Objects, Constructors and Inheritance

This section tests object lifecycle and the order in which constructors and destructors run.

Constructors and destructors

• A constructor has the same name as the class, no return type, and runs automatically when an object is created.
• Types: default (no arguments), parameterised, and copy constructor (ClassName(const ClassName &)).
• If you declare any constructor, the compiler no longer supplies a default one.
• A destructor is ~ClassName(), takes no arguments, and runs when the object's lifetime ends.
• Constructors cannot be virtual; destructors in a polymorphic base should be virtual.

Inheritance

• Forms: single, multiple, multilevel, hierarchical, hybrid.
• Access modes public, protected, private change how base members appear in the derived class.
• Construction order: base constructor first, then derived; destruction reverses this.
• A derived constructor can pass arguments to the base using an initialiser list.

Worked example

class A { public: A(){ cout << 'A'; } ~A(){ cout << 'a'; } }; class B : public A { public: B(){ cout << 'B'; } ~B(){ cout << 'b'; } }; Creating and destroying one B object prints ABba. Reasoning: base is constructed before derived, and destruction is reverse - derived destroyed first, then base.

Traps

• Giving a constructor a return type.
• Assuming a parameterised constructor still provides a zero-argument default.
• Reversing constructor/destructor order.
• Thinking private base members become accessible under public inheritance; they remain inaccessible directly.
• Forgetting the virtual destructor when deleting derived objects through a base pointer.