Object interaction in C++ - Step-by-Step Execution

Concept Flow - Object interaction

Create Object A

↓

Create Object B

↓

Object A calls method on Object B

↓

Object B processes and returns result

↓

Object A uses result

↓

End

This flow shows how one object creates or accesses another object, calls its method, and uses the returned result.

Execution Sample

C++

#include <iostream>

class Printer {
public:
  void printMessage() { std::cout << "Hello from Printer!\n"; }
};

class Manager {
  Printer printer;
public:
  void start() { printer.printMessage(); }
};

int main() {
  Manager mgr;
  mgr.start();
  return 0;
}

Manager object calls a method on its Printer object to print a message.

Execution Table

Step	Action	Object Involved	Method Called	Output
1	Create Manager object	Manager	Constructor
2	Manager creates Printer object internally	Printer	Constructor
3	Manager calls start()	Manager	start()
4	start() calls printer.printMessage()	Printer	printMessage()
5	printMessage() outputs message	Printer	printMessage()	Hello from Printer!
6	start() finishes	Manager	start()
7	main() finishes	main

💡 Program ends after main() returns 0

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
mgr (Manager object)	none	exists	exists with printer object	exists with printer object	exists with printer object
printer (Printer object inside Manager)	none	none	exists	exists	exists

Key Moments - 3 Insights

Why does Manager have a Printer object inside it?

Because Manager creates Printer as a member variable (see step 2 in execution_table), so it can call its methods directly.

When Manager calls start(), how does the message get printed?

start() calls printer.printMessage() (step 4), which outputs the message (step 5).

Does main() directly call printMessage()?

No, main() calls mgr.start() (step 3), and start() calls printMessage().

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the Printer object get created?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Check the 'Action' column for when Printer is created.

At which step does the message "Hello from Printer!" first appear in the output?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the 'Output' column for when the message is printed.

If Manager did not have a Printer object, what would happen when start() tries to call printMessage()?

A It would print the message anyway.

B The program would crash at runtime.

C There would be a compile error because printer does not exist.

D Nothing would happen.

💡 Hint

Think about object existence before method calls, see variable_tracker for printer.

Concept Snapshot

Object interaction means one object uses another object's methods.
Objects can be members inside other objects.
One object calls methods on another to get work done.
This is done by creating objects and calling their methods.
Example: Manager has Printer; Manager calls Printer's printMessage().

Full Transcript

This example shows how objects interact in C++. First, a Manager object is created. Inside Manager, a Printer object is created as a member variable. When Manager's start() method is called, it calls the printMessage() method on its Printer object. This prints the message "Hello from Printer!" to the screen. The main function creates a Manager and calls start(), which triggers the message printing. The execution table traces each step: creating objects, calling methods, and outputting text. The variable tracker shows how the Manager and Printer objects exist through the program. Key moments clarify why Manager has Printer inside and how method calls flow. The quiz tests understanding of object creation, method calls, and what happens if an object is missing. This helps beginners see how objects work together in a simple program.