C++ Virtual Functions

🧠 C++ Virtual Functions

A virtual function in C++ is a member function that is declared using the virtual keyword and is overridden in a derived class.
It supports run-time polymorphism, meaning the function call is resolved at runtime based on the object type, not the pointer type.

1. Why Virtual Functions?

Without virtual functions, C++ uses compile-time binding (static binding).
With virtual functions, C++ uses dynamic binding.

2. Basic Syntax

3. Virtual Function Example

✔ Function call decided at runtime
✔ Based on object type

4. Without `virtual` (No Polymorphism)

❌ Wrong behavior for polymorphism

5. Virtual Function Rules

Declared in base class
Overridden in derived class
Accessed using base class pointer/reference
Must have same signature

6. `override` Keyword (Best Practice)

✔ Catches signature mistakes
✔ Improves readability

7. Virtual Destructor (Very Important)

✔ Ensures correct destructor call when deleting derived object via base pointer

8. Pure Virtual Function (Abstract Class)

✔ Class becomes abstract
✔ Cannot create object of abstract class

9. Virtual Table (vtable) – Concept

Compiler creates a vtable for classes with virtual functions
Base pointer uses vtable to call correct function at runtime
Slight memory overhead

10. Virtual Function in Multilevel Inheritance

class A {
public:
virtual void show() {
cout << "A";
}
};class B : public A {
public:
void show() {
cout << "B";
}
};

class C : public B {
public:
void show() {
cout << "C";
}
};



A* a = new C();
a->show(); // Output: C

class A {

public:

virtual void show() {

cout << "A";

}

};class B : public A {

public:

void show() {

cout << "B";

}

};

class C : public B {

public:

void show() {

cout << "C";

}

};

A* a = new C();

a->show(); // Output: C

❌ Common Mistakes

🔁 Virtual vs Non-Virtual

Virtual	Non-Virtual
Runtime binding	Compile-time binding
Polymorphism	No polymorphism
Slightly slower	Faster
Uses vtable	No vtable

📌 Summary

Virtual functions enable runtime polymorphism
Declared using virtual
Use base pointer to derived object
override improves safety
Virtual destructors are critical

C++ Virtual Functions