Classes and objects in C++ - Step-by-Step Execution

Concept Flow - Classes and objects

Define class blueprint

↓

Create object from class

↓

Access object properties

↓

Call object methods

↓

Use object data and behavior

This flow shows how a class is defined as a blueprint, then an object is created from it, and finally the object's properties and methods are used.

Execution Sample

C++

#include <iostream>

class Car {
public:
  int speed;
  void honk() { std::cout << "Beep!\n"; }
};

int main() {
  Car myCar{};
  myCar.speed = 50;
  myCar.honk();
  return 0;
}

Defines a Car class with a speed property and honk method, creates an object, sets speed, and calls honk.

Execution Table

Step	Action	Evaluation	Result
1	Define class Car with int speed and method honk()	Class blueprint created	Car class ready
2	Create object myCar of type Car	Object myCar created	myCar exists with default speed
3	Set myCar.speed = 50	Assign 50 to speed property	myCar.speed is 50
4	Call myCar.honk()	Execute honk method	Output: Beep!
5	End of program	No more instructions	Program ends

💡 Program ends after calling honk method and no more instructions remain

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
myCar.speed	undefined	0 (default)	50	50

Key Moments - 2 Insights

Why does myCar.speed have a default value before assignment?

In step 2, the object myCar is created and its int speed is initialized to 0 by default, as shown in the variable_tracker after Step 2.

What happens when we call myCar.honk()?

At step 4, calling myCar.honk() runs the method which prints "Beep!" to the output, as shown in the execution_table Result column.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of myCar.speed after Step 3?

A undefined

B 50

C 0

D Beep!

💡 Hint

Check variable_tracker row for myCar.speed after Step 3

At which step is the Car class blueprint created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the execution_table Action column for class definition

If we did not call myCar.honk(), what would be missing in the execution_table?

A Step 3 setting speed

B Step 2 object creation

C Step 4 output "Beep!"

D Step 1 class definition

💡 Hint

Check which step produces the output "Beep!" in the execution_table Result

Concept Snapshot

class ClassName {
public:
  // properties
  // methods
};

Create object: ClassName obj;
Access: obj.property, obj.method();
Class is blueprint; object is instance with data and behavior.

Full Transcript

This example shows how to define a class named Car with a property speed and a method honk. The program creates an object myCar from the Car class. Initially, myCar.speed is set to 0 by default. Then the program sets myCar.speed to 50. Finally, it calls myCar.honk(), which prints "Beep!" to the output. The execution table traces each step: defining the class, creating the object, setting the speed, calling the method, and ending the program. The variable tracker shows how myCar.speed changes from undefined to 0 to 50. Key moments clarify why speed has a default value and what calling honk does. The quiz tests understanding of these steps and values. This visual trace helps beginners see how classes and objects work step-by-step in C++.