Runtime Polymorphism

Runtime polymorphism, also known as dynamic polymorphism, represents one of the core pillars of object-oriented programming. It enables a program to decide which method implementation to execute based on the actual type of an object during program execution, rather than at compile time.

Core Mechanisms

Virtual Functions

The primary mechanism enabling runtime polymorphism is the virtual function system:

1. Base class declares virtual methods
2. Derived classes override these methods
3. Program maintains a virtual function table (vtable) to track method implementations

class Animal {
    public:
        virtual void makeSound() = 0;
};

class Dog : public Animal {
    public:
        void makeSound() { /* bark implementation */ }
};

Key Concepts

Late Binding

Dynamic dispatch (late binding) is the process where:

• Method calls are resolved at runtime
• The program determines the correct method implementation based on the object's actual type
• The vtable is consulted to find the appropriate method address

Base Class Pointers

Runtime polymorphism typically involves:

• Using base class pointers/references to refer to derived class objects
• Allowing collections of different derived types through common interface
• Supporting dependency injection and flexible designs

Benefits and Applications

Advantages

1. Improved code flexibility and maintainability
2. Support for the Open-Closed Principle
3. Enhanced code reusability
4. Reduced coupling between components

Common Use Cases

• Plugin architecture systems
• Framework development
• Design patterns implementation (especially Strategy Pattern and Factory Pattern)

Performance Considerations

Runtime polymorphism introduces some overhead:

• Additional memory for vtables
• Extra indirection for method calls
• Cache coherence implications

Best Practices

1. Use virtual destructors in base classes
2. Prefer virtual functions to conditional logic
3. Consider SOLID principles when designing class hierarchies
4. Be mindful of object slicing when passing objects by value

Comparison with Static Polymorphism

Runtime polymorphism differs from compile-time polymorphism in several ways:

• Resolution timing (runtime vs. compile-time)
• Performance characteristics
• Flexibility vs. type safety tradeoffs
• Implementation mechanisms

Common Pitfalls

1. Forgetting virtual destructors
2. Incorrect override specifications
3. Object slicing issues
4. Memory leak risks with improper cleanup

Runtime polymorphism remains a crucial feature in modern object-oriented systems, enabling flexible and maintainable code structures while supporting key design patterns and architectural approaches.