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06_virtual_functions.cpp
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#include <iostream>
#include <string>
#include <vector>
using namespace std;
/*
* PART 6: VIRTUAL FUNCTIONS & POLYMORPHISM
* =========================================
* Deep dive into virtual functions and runtime polmorphism:
* - What is polymorphism?
* - How virtual functions work (vtable/vptr)
* - Pure virtual functions
* - Abstract classes
* - Runtime vs Compile-time polymorphism
*/
// ============================================================================
// EXAMPLE 1: What is Polymorphism?
// ============================================================================
class Shape {
public:
virtual void draw() {
cout << "Drawing Shape" << endl;
}
virtual double area() {
return 0.0;
}
virtual ~Shape() { }
};
class Circle : public Shape {
private:
double radius;
public:
Circle(double r) : radius(r) { }
void draw() override {
cout << "Drawing Circle with radius " << radius << endl;
}
double area() override {
return 3.14159 * radius * radius;
}
};
class Rectangle : public Shape {
private:
double width, height;
public:
Rectangle(double w, double h) : width(w), height(h) { }
void draw() override {
cout << "Drawing Rectangle " << width << "x" << height << endl;
}
double area() override {
return width * height;
}
};
void testPolymorphism() {
cout << "\n╔════════════════════════════════════════╗" << endl;
cout << "║ WHAT IS POLYMORPHISM? ║" << endl;
cout << "╚════════════════════════════════════════╝" << endl;
// One interface, multiple implementations!
vector<Shape*> shapes;
shapes.push_back(new Circle(5.0));
shapes.push_back(new Rectangle(4.0, 6.0));
shapes.push_back(new Circle(3.0));
cout << "\nDrawing all shapes polymorphically:" << endl;
for (Shape* shape : shapes) {
shape->draw(); // ✅ Calls correct version based on actual type!
cout << "Area: " << shape->area() << endl;
}
// Cleanup
for (Shape* shape : shapes) {
delete shape;
}
cout << "\n📝 Polymorphism: 'Many forms' - one interface, multiple implementations" << endl;
}
// ============================================================================
// EXAMPLE 2: Pure Virtual Functions & Abstract Classes
// ============================================================================
class Animal {
public:
// Pure virtual function (= 0 makes it pure virtual)
virtual void sound() = 0;
// Regular virtual function
virtual void eat() {
cout << "Animal is eating" << endl;
}
// Virtual destructor (always in abstract classes!)
virtual ~Animal() { }
};
class Dog : public Animal {
public:
void sound() override { // MUST implement pure virtual
cout << "Woof!" << endl;
}
void eat() override {
cout << "Dog is eating bones" << endl;
}
};
class Cat : public Animal {
public:
void sound() override { // MUST implement pure virtual
cout << "Meow!" << endl;
}
// eat() not overridden, uses Animal's version
};
void testAbstractClasses() {
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ PURE VIRTUAL FUNCTIONS & ABSTRACT CLASSES ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
// Animal a; // ❌ ERROR! Cannot instantiate abstract class
Animal* animals[2];
animals[0] = new Dog();
animals[1] = new Cat();
cout << "\nCalling polymorphic functions:" << endl;
for (int i = 0; i < 2; i++) {
animals[i]->sound();
animals[i]->eat();
cout << "---" << endl;
}
delete animals[0];
delete animals[1];
cout << "\n📝 Pure virtual (= 0) makes class abstract" << endl;
cout << " Derived classes MUST implement pure virtual functions" << endl;
}
// ============================================================================
// EXAMPLE 3: How Virtual Functions Work (vtable/vptr)
// ============================================================================
class Base {
public:
virtual void func1() { cout << "Base::func1()" << endl; }
virtual void func2() { cout << "Base::func2()" << endl; }
void func3() { cout << "Base::func3()" << endl; } // NOT virtual
virtual ~Base() { }
};
class Derived : public Base {
public:
void func1() override { cout << "Derived::func1()" << endl; }
void func2() override { cout << "Derived::func2()" << endl; }
void func3() { cout << "Derived::func3()" << endl; }
};
void testVtableMechanism() {
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ HOW VIRTUAL FUNCTIONS WORK (vtable/vptr) ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
cout << "\nSize comparison:" << endl;
cout << "sizeof(int) = " << sizeof(int) << " bytes" << endl;
class NoVirtual {
int x;
};
cout << "sizeof(NoVirtual) = " << sizeof(NoVirtual) << " bytes" << endl;
class WithVirtual {
int x;
virtual void func() { }
};
cout << "sizeof(WithVirtual) = " << sizeof(WithVirtual) << " bytes" << endl;
cout << " (Extra " << sizeof(WithVirtual) - sizeof(NoVirtual)
<< " bytes for vptr!)" << endl;
cout << "\n--- Virtual function calls ---" << endl;
Base* b = new Derived();
b->func1(); // ✅ Virtual - calls Derived version
b->func2(); // ✅ Virtual - calls Derived version
b->func3(); // ❌ NOT virtual - calls Base version
delete b;
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ INTERNAL MECHANISM: ║" << endl;
cout << "║ ║" << endl;
cout << "║ Each object with virtual functions has: ║" << endl;
cout << "║ • vptr (virtual pointer) - points to vtable ║" << endl;
cout << "║ ║" << endl;
cout << "║ Each class has: ║" << endl;
cout << "║ • vtable (virtual table) - array of function ptrs║" << endl;
cout << "║ ║" << endl;
cout << "║ When calling b->func1(): ║" << endl;
cout << "║ 1. Follow b's vptr to find vtable ║" << endl;
cout << "║ 2. Look up func1 in vtable ║" << endl;
cout << "║ 3. Call the function pointer found ║" << endl;
cout << "║ ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
}
// ============================================================================
// EXAMPLE 4: Runtime vs Compile-time Polymorphism
// ============================================================================
// Compile-time polymorphism (Function Overloading)
class Calculator {
public:
int add(int a, int b) {
return a + b;
}
double add(double a, double b) {
return a + b;
}
int add(int a, int b, int c) {
return a + b + c;
}
};
// Runtime polymorphism (Virtual Functions)
class Payment {
public:
virtual void processPayment(double amount) = 0;
virtual ~Payment() { }
};
class CreditCard : public Payment {
public:
void processPayment(double amount) override {
cout << "Processing credit card payment: $" << amount << endl;
}
};
class PayPal : public Payment {
public:
void processPayment(double amount) override {
cout << "Processing PayPal payment: $" << amount << endl;
}
};
void testPolymorphismTypes() {
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ RUNTIME vs COMPILE-TIME POLYMORPHISM ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
cout << "\n--- Compile-time (Function Overloading) ---" << endl;
Calculator calc;
cout << "add(2, 3) = " << calc.add(2, 3) << endl;
cout << "add(2.5, 3.7) = " << calc.add(2.5, 3.7) << endl;
cout << "add(1, 2, 3) = " << calc.add(1, 2, 3) << endl;
cout << "Decision made at: COMPILE TIME" << endl;
cout << "\n--- Runtime (Virtual Functions) ---" << endl;
Payment* payment;
// Decision made at RUNTIME based on user input
int choice = 1; // Simulate user choice
if (choice == 1) {
payment = new CreditCard();
} else {
payment = new PayPal();
}
payment->processPayment(100.0);
cout << "Decision made at: RUNTIME" << endl;
delete payment;
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ COMPILE-TIME: Function selected during compilation║" << endl;
cout << "║ RUNTIME: Function selected during execution ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
}
// ============================================================================
// EXAMPLE 5: Virtual Function Rules & Best Practices
// ============================================================================
class Rules_Base {
public:
// Rule 1: Virtual destructor if class has virtual functions
virtual ~Rules_Base() { }
// Rule 2: Pure virtual for interface
virtual void mustImplement() = 0;
// Rule 3: Virtual with default implementation
virtual void canOverride() {
cout << "Default implementation" << endl;
}
// Rule 4: Non-virtual for fixed behavior
void dontOverride() {
cout << "Fixed behavior - don't override" << endl;
}
// Rule 5: Final prevents further overriding
virtual void lastLevel() final {
cout << "Cannot override this in derived classes" << endl;
}
};
class Rules_Derived : public Rules_Base {
public:
void mustImplement() override {
cout << "Implemented in derived" << endl;
}
void canOverride() override {
cout << "Overridden in derived" << endl;
}
// void lastLevel() override { } // ❌ ERROR! Marked final in base
};
void testVirtualRules() {
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ VIRTUAL FUNCTION RULES & BEST PRACTICES ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
Rules_Base* obj = new Rules_Derived();
obj->mustImplement();
obj->canOverride();
obj->dontOverride();
obj->lastLevel();
delete obj;
cout << "\n📝 Rules:" << endl;
cout << " 1. Virtual destructor if class has any virtual function" << endl;
cout << " 2. Pure virtual (= 0) for interface/abstract behavior" << endl;
cout << " 3. Virtual with body for optional override" << endl;
cout << " 4. Non-virtual for fixed behavior" << endl;
cout << " 5. 'final' keyword prevents further overriding" << endl;
}
// ============================================================================
// EXAMPLE 6: Real-World Example - Plugin System
// ============================================================================
class Plugin {
protected:
string name;
public:
Plugin(string n) : name(n) { }
virtual void initialize() = 0;
virtual void execute() = 0;
virtual void cleanup() = 0;
string getName() { return name; }
virtual ~Plugin() { }
};
class LoggerPlugin : public Plugin {
public:
LoggerPlugin() : Plugin("Logger") { }
void initialize() override {
cout << "[" << name << "] Initializing logger..." << endl;
}
void execute() override {
cout << "[" << name << "] Logging data..." << endl;
}
void cleanup() override {
cout << "[" << name << "] Closing log files..." << endl;
}
};
class DatabasePlugin : public Plugin {
public:
DatabasePlugin() : Plugin("Database") { }
void initialize() override {
cout << "[" << name << "] Connecting to database..." << endl;
}
void execute() override {
cout << "[" << name << "] Querying data..." << endl;
}
void cleanup() override {
cout << "[" << name << "] Closing connections..." << endl;
}
};
void testRealWorldExample() {
cout << "\n╔════════════════════════════════════════════════════╗" << endl;
cout << "║ REAL-WORLD EXAMPLE: PLUGIN SYSTEM ║" << endl;
cout << "╚════════════════════════════════════════════════════╝" << endl;
vector<Plugin*> plugins;
plugins.push_back(new LoggerPlugin());
plugins.push_back(new DatabasePlugin());
cout << "\n--- Loading plugins ---" << endl;
for (Plugin* plugin : plugins) {
plugin->initialize();
}
cout << "\n--- Running plugins ---" << endl;
for (Plugin* plugin : plugins) {
plugin->execute();
}
cout << "\n--- Unloading plugins ---" << endl;
for (Plugin* plugin : plugins) {
plugin->cleanup();
delete plugin;
}
cout << "\n💡 Polymorphism allows dynamic plugin loading without knowing types at compile time!" << endl;
}
// ============================================================================
// MAIN
// ============================================================================
int main() {
cout << "\n";
cout << "╔════════════════════════════════════════════════════════════════╗" << endl;
cout << "║ ║" << endl;
cout << "║ VIRTUAL FUNCTIONS & POLYMORPHISM ║" << endl;
cout << "║ ║" << endl;
cout << "╚════════════════════════════════════════════════════════════════╝" << endl;
// Example 1: What is polymorphism
testPolymorphism();
// Example 2: Abstract classes
testAbstractClasses();
// Example 3: vtable mechanism
testVtableMechanism();
// Example 4: Runtime vs compile-time
testPolymorphismTypes();
// Example 5: Virtual rules
testVirtualRules();
// Example 6: Real-world example
testRealWorldExample();
cout << "\n";
cout << "╔════════════════════════════════════════════════════════════════╗" << endl;
cout << "║ INTERVIEW TIPS ║" << endl;
cout << "╠════════════════════════════════════════════════════════════════╣" << endl;
cout << "║ ║" << endl;
cout << "║ Q: What is polymorphism? ║" << endl;
cout << "║ A: 'Many forms' - ability to call derived class methods ║" << endl;
cout << "║ through base class pointer/reference at runtime ║" << endl;
cout << "║ ║" << endl;
cout << "║ Q: How do virtual functions work internally? ║" << endl;
cout << "║ A: Via vtable (virtual table) and vptr (virtual pointer) ║" << endl;
cout << "║ Each object has vptr pointing to class's vtable ║" << endl;
cout << "║ ║" << endl;
cout << "║ Q: What's a pure virtual function? ║" << endl;
cout << "║ A: virtual void func() = 0; Makes class abstract ║" << endl;
cout << "║ Derived classes MUST implement it ║" << endl;
cout << "║ ║" << endl;
cout << "║ Q: Compile-time vs Runtime polymorphism? ║" << endl;
cout << "║ A: Compile: Function overloading ║" << endl;
cout << "║ Runtime: Virtual functions (more flexible) ║" << endl;
cout << "║
57A6
║" << endl;
cout << "║ Q: Cost of virtual functions? ║" << endl;
cout << "║ A: 8 bytes per object (vptr) + slight call overhead ║" << endl;
cout << "║ ║" << endl;
cout << "╚════════════════════════════════════════════════════════════════╝" << endl;
return 0;
}
/*
* SUMMARY - Virtual Functions & Polymorphism:
* ============================================
*
* POLYMORPHISM:
* - "Many forms" - one interface, multiple implementations
* - Base pointer can call derived class methods
* - Enables writing flexible, extensible code
*
* VIRTUAL FUNCTIONS:
* - Enable runtime polymorphism
* - Use 'virtual' keyword in base class
* - Use 'override' keyword in derived class (C++11)
* - Always make destructor virtual if class has virtual functions
*
* PURE VIRTUAL FUNCTIONS:
* - Syntax: virtual void func() = 0;
* - Makes class abstract (cannot instantiate)
* - Derived classes MUST implement
* - Used for interfaces/contracts
*
* INTERNAL MECHANISM:
* - vtable: Array of function pointers per class
* - vptr: Pointer in each object pointing to class's vtable
* - Runtime lookup: Follow vptr → find function in vtable → call it
* - Cost: 8 bytes per object + slight overhead
*
* TWO TYPES OF POLYMORPHISM:
* 1. Compile-time: Function overloading, templates
* 2. Runtime: Virtual functions (more flexible)
*
* BEST PRACTICES:
* - Always use virtual destructor in polymorphic base classes
* - Use 'override' keyword for clarity and safety
* - Use 'final' to prevent further overriding if needed
* - Pure virtual for interfaces, virtual for optional override
*/