Base class pointers in C++ - Step-by-Step Execution

Concept Flow - Base class pointers

Create Derived object

↓

Assign Derived object address to Base pointer

↓

Use Base pointer to access members

↓

Call functions via Base pointer

↓

If virtual, Derived version runs

↓

If not virtual, Base version runs

A pointer of base class type can hold address of derived class object. Calling functions depends on virtual keyword.

Execution Sample

C++

#include <iostream>

class Base {
public:
  void show() { std::cout << "Base show\n"; }
};

class Derived : public Base {
public:
  void show() { std::cout << "Derived show\n"; }
};

int main() {
  Derived d;
  Base* ptr = &d;
  ptr->show();
  return 0;
}

Shows how a base class pointer calls a function when pointing to a derived object.

Execution Table

Step	Action	Pointer Type	Object Pointed To	Function Called	Output
1	Create Derived object d	-	Derived object d	-	-
2	Assign address of d to Base pointer ptr	Base*	Derived object d	-	-
3	Call ptr->show()	Base*	Derived object d	Base::show()	Base show
4	Program ends	-	-	-	-

💡 Function called depends on pointer type and virtual keyword; here show() is non-virtual, so Base::show() runs

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
d	uninitialized	Derived object created	Derived object created	Derived object created	Derived object created
ptr	uninitialized	uninitialized	points to d (Derived)	points to d (Derived)	points to d (Derived)

Key Moments - 2 Insights

Why does ptr->show() call Base::show() and not Derived::show()?

Because show() is not declared virtual in Base, the function called depends on pointer type, not object type. See execution_table step 3.

Can a Base class pointer hold address of a Derived class object?

Yes, a Base class pointer can point to a Derived object. This is shown in execution_table step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what function is called at step 3?

A Derived::show()

B Base::show()

C No function called

D Compiler error

💡 Hint

Check the 'Function Called' column at step 3 in execution_table.

At which step does the Base pointer start pointing to the Derived object?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Pointer Type' and 'Object Pointed To' columns in execution_table.

If show() was declared virtual in Base, what would ptr->show() call at step 3?

A Base::show()

B No function called

C Derived::show()

D Compiler error

💡 Hint

Virtual functions call the derived version when accessed via base pointer.

Concept Snapshot

Base class pointers can hold addresses of derived objects.
Calling functions via base pointer calls base version unless function is virtual.
Virtual functions enable polymorphism: derived version runs.
Non-virtual functions call base version regardless of object type.
Use base pointers for flexible code with inheritance.

Full Transcript

This example shows how a pointer of base class type can point to a derived class object. We create a Derived object d, then assign its address to a Base class pointer ptr. When we call ptr->show(), since show() is not virtual, the Base class version runs, printing 'Base show'. If show() were virtual, the Derived version would run instead. This demonstrates how base class pointers work and how virtual functions enable polymorphism.