Base and derived classes in C++ - Step-by-Step Execution

Concept Flow - Base and derived classes

Define Base Class

↓

Define Derived Class

↓

Create Derived Object

↓

Call Derived Method

↓

Call Base Method (inherited)

↓

Use Derived and Base Features

First, we define a base class, then a derived class inherits from it. We create an object of the derived class and use both its own and inherited features.

Execution Sample

C++

#include <iostream>

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

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

int main() {
  Derived d;
  d.greet();
  d.Base::greet();
  return 0;
}

This code shows a base class and a derived class both with a greet method. The derived class object calls its own greet and the base class greet.

Execution Table

Step	Action	Evaluation	Output
1	Define Base class with greet()	Base::greet() ready
2	Define Derived class inheriting Base, override greet()	Derived::greet() ready
3	Create Derived object d	Object d created
4	Call d.greet()	Calls Derived::greet()	Hello from Derived
5	Call d.Base::greet()	Calls Base::greet() explicitly	Hello from Base
6	Program ends	No more actions

💡 Program ends after calling both greet methods on Derived object

Variable Tracker

Variable	Start	After Creation	After Calls	Final
d (Derived object)	undefined	Created	Used to call greet methods	Exists until program ends

Key Moments - 2 Insights

Why does d.greet() call Derived's greet, not Base's?

Because Derived overrides greet(), so calling d.greet() uses Derived's version (see execution_table step 4).

How can we call Base's greet() from Derived object?

By specifying d.Base::greet(), we tell the program to use Base's greet explicitly (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, what is printed at step 4 when d.greet() is called?

A Hello from Base

B Hello from Derived

C No output

D Compilation error

💡 Hint

Check the Output column at step 4 in the execution_table

At which step does the program explicitly call the Base class greet() method?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look for the action mentioning d.Base::greet() in the execution_table

If Derived did not override greet(), what would d.greet() print?

A Hello from Derived

B No output

C Hello from Base

D Compilation error

💡 Hint

Think about inheritance and method overriding shown in the execution_table steps 4 and 5

Concept Snapshot

Base and derived classes:
- Define base class with methods
- Define derived class inheriting base
- Derived can override base methods
- Derived object uses overridden methods by default
- Use Base::method() to call base version
- Supports code reuse and extension

Full Transcript

This example shows how a derived class inherits from a base class in C++. The base class has a greet method. The derived class overrides this greet method. When we create an object of the derived class and call greet, it uses the derived version. We can also call the base class greet explicitly using the scope operator. This demonstrates basic inheritance and method overriding in C++.