Polymorphism

Virtual Functions

• Definition: A function that gets called based on the object's type rather than the pointer/reference type during upcasting
• Purpose: Enables runtime polymorphism (dynamic method dispatch)
• Declaration: Use virtual keyword in the base class
• Polymorphic class: Any class with at least one virtual function
• Inheritance of virtual property: If a base class function is virtual, the derived class function with the same signature is automatically virtual

Function Overriding Rules

1. Function must exist in both base and derived classes (different scopes)
2. Signatures (including return type) must be identical
3. The base class function must be virtual

Binding Mechanisms

• Early Binding (Static/Compile-time):
  • Function calls resolved at compile time
  • Occurs when calling any function on an object
  • Occurs when calling non-virtual functions on pointers/references
• Late Binding (Dynamic/Runtime):
  • Function calls resolved at runtime
  • Occurs when calling virtual functions on pointers/references
  • Enables polymorphic behavior

Virtual Function Implementation (vptr and vtable)

• vtable (Virtual Function Table):
  • Created per class with virtual functions
  • Array of function pointers to virtual functions
  • Compiler-generated data structure
• vptr (Virtual Function Pointer):
  • Implicit pointer added to objects of classes with virtual functions
  • Points to the class's vtable
  • Usually positioned at the start of the object
  • Added once per object
  • Automatically managed by the compiler
  • Increases object size by 2/4/8 bytes depending on the architecture
• Memory Impact: Size of object = size of non-static data members + vptr size (if the class has virtual functions)

Virtual Function Design Guidelines

• Non-virtual function: When base class implementation is 100% complete