Debugging Grimoire

C++ Virtual Function Behavior

Virtual Function Selection

• Base classes typically define a virtual function.
• Derived classes override these functions.
• Pointer to base can point at base or derived class objects.
• Function selected depends on object type, not pointer type.
• If no derived override exists, the base class function is used.

Virtual & Overloaded Function Selection

• Overloaded member functions are selected at compile-time based on signature.
• They can have different return types.
• Once a function is declared virtual, it stays virtual in derived classes.
• The virtual keyword is not required in the redefinition, but it's clearer if included.

Example Code

#include <iostream>
#include <ostream>

class B {
public:
  int i;
  virtual void print_i() const { std::cout << i << " inside B" << std::endl; }
};

class D : public B {
public:
  void print_i() const { std::cout << i << " inside D" << std::endl; }
};

int main() {
  B b;
  B *pb = &b; // point at a B object
  D f;

  f.i = 1 + (b.i = 1);
  pb->print_i(); // Call B::print_i()
  pb = &f;       // point at D object
  pb->print_i(); // Call D::print_i()
}

Output

1 inside B
2 inside D

Analysis

• First pb->print_i() calls B::print_i() because pb points to a B object.
• Second pb->print_i() calls D::print_i() because pb now points to a D object.
• Function selection happens dynamically at runtime based on the actual object.

Object-Oriented Principles

• Abstract Data Types (ADTs), inheritance, and polymorphism allow treating different class objects through a common interface.
• Virtual functions enable run-time method resolution—true polymorphism.
• This dynamic dispatch is at the heart of OOP.