🌀 C++ Polymorphism – Compile-Time & Run-Time Explained

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🧲 Introduction – Why Polymorphism Matters in C++

Polymorphism is a powerful feature in object-oriented programming in C++ that enables a single function or interface to behave differently based on the object or context. It enhances flexibility, reusability, and maintainability in complex applications.

🎯 In this guide, you’ll learn:

• What polymorphism is and its types
• Compile-time vs run-time polymorphism
• Virtual functions, method overriding, and use cases
• Code examples with output and best practices

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🔍 What Is Polymorphism?

The word “polymorphism” means “many forms.” In C++, it allows functions or methods to behave differently based on:

• Object type (Run-time Polymorphism)
• Arguments passed (Compile-time Polymorphism)

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⚙️ Types of Polymorphism in C++

Type	Description	Key Feature
Compile-time	Function overloading or operator overloading	Early binding
Run-time	Base pointer calling derived function	Late binding (virtual)

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💻 Code Examples – With Output

✅ Example 1: Compile-Time Polymorphism (Function Overloading)

#include <iostream>
using namespace std;

class Print {
public:
    void show(int i) {
        cout << "Integer: " << i << endl;
    }
    void show(string s) {
        cout << "String: " << s << endl;
    }
};

int main() {
    Print p;
    p.show(10);
    p.show("Hello");
    return 0;
}

🟢 Output:

Integer: 10
String: Hello

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✅ Example 2: Run-Time Polymorphism (Virtual Function)

#include <iostream>
using namespace std;

class Shape {
public:
    virtual void draw() {
        cout << "Drawing Shape" << endl;
    }
};

class Circle : public Shape {
public:
    void draw() override {
        cout << "Drawing Circle" << endl;
    }
};

int main() {
    Shape* s;
    Circle c;
    s = &c;
    s->draw();  // Calls Circle’s draw()
    return 0;
}

🟢 Output:

Drawing Circle

🔍 Explanation:

• draw() is a virtual function.
• Even though s is a Shape*, it calls Circle::draw() due to runtime polymorphism.

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🧬 Virtual Functions – The Backbone of Polymorphism

• Declared using the virtual keyword
• Enables method overriding in derived classes
• Supports dynamic dispatch (late binding)

✅ Virtual Function Syntax:

class Base {
public:
    virtual void display();
};

✅ Override in Derived Class:

class Derived : public Base {
public:
    void display() override;
};

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🧠 Polymorphism Comparison Table

Feature	Compile-Time (Static)	Run-Time (Dynamic)
Examples	Function Overloading, Templates	Virtual Functions, Method Overriding
Decision made at	Compile time	Run time
Performance	Faster	Slightly slower
Flexibility	Limited	High
Keywords	None (implicit)	virtual, override

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💡 Best Practices & Tips

📘 Best Practice: Always use virtual in base classes when methods are meant to be overridden.

💡 Tip: Use override keyword in derived classes to prevent accidental mismatches.

⚠️ Pitfall: Forgetting virtual leads to object slicing—base method gets called instead of derived.

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🛠️ Use Cases for Polymorphism

🔁 Game Engines: Entity → Player, Enemy, NPC, all override update()
🧾 Billing Systems: Invoice → GSTInvoice, DiscountedInvoice with override methods
🎨 Graphics Libraries: Shape → Circle, Rectangle, Triangle with draw() override
📦 Plugin Frameworks: Interface-based polymorphism to load modules dynamically

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📌 Summary – Recap & Next Steps

🔍 Key Takeaways:

• Polymorphism allows flexible and extensible design
• C++ supports both compile-time (overloading) and run-time (virtual) polymorphism
• Use virtual and override wisely for safe and correct inheritance behavior

⚙️ Real-World Relevance:
Used in rendering engines, data pipelines, simulations, frameworks, and plugin systems where behavior differs but interfaces remain unified.

✅ Next Steps:

• Learn about Abstraction and Encapsulation
• Explore Virtual Destructors and VTables

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❓FAQ – C++ Polymorphism

❓Is polymorphism only used with inheritance?
✅ For run-time polymorphism, yes—it relies on virtual functions in inheritance.

❓Is function overloading the same as polymorphism?
✅ Yes, function overloading is compile-time (static) polymorphism.

❓Can constructors be virtual?
❌ No. Constructors cannot be virtual, but destructors should be if the class is used polymorphically.

❓What is object slicing?
⚠️ It occurs when a derived object is assigned to a base object—not a pointer or reference—causing loss of derived-specific data.

❓What happens if I forget virtual in the base class?
⛔ The base version of the function is called, even when using a derived class object—breaking polymorphism.

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