This repository contains a single C program that demonstrates how to implement core Object-Oriented Programming (OOP) concepts in standard C, which does not natively support them.

This project is an academic exercise to show how OOP-like systems (such as GObject or parts of the Linux kernel) can be built from the ground up. It manually implements the "machinery" that a C++ compiler would normally handle automatically.

This program simulates all major OOP principles using only C's built-in features:

• Encapsulation: Achieved by using struct to bundle data. True encapsulation (hiding data) can be achieved by forward-declaring struct in a public header file and only defining it in the source file, forcing users to interact via public functions.
• Inheritance: Simulated using struct composition. The "derived" struct (e.g., Warrior) contains the "base" struct (e.g., Entity) as its very first member. This guarantees that a pointer to Warrior can be safely cast to a pointer to Entity.
• Polymorphism (Dynamic Dispatch): Implemented by creating a manual v-table (virtual table).
  • The Entity_vtable is a struct containing function pointers (attack, displayStatus, destroy).
  • Each "base" Entity struct contains a pointer to a v-table.
  • "Constructors" (like Warrior_create) assign this pointer to the specific v-table for that type (e.g., &WARRIOR_VTABLE).
  • When we call entity->vtable->attack(entity), it dynamically calls the correct function (Warrior_attack or Mage_attack) at runtime, based on which v-table the object points to.

This program is a single file and has no external dependencies. You can compile it with any standard C compiler, such as gcc.

git clone https://github.com/DEMiHAT/c-vtable-demo
cd c-vtable-demo

# 'gcc' is the compiler
# 'oop_in_c.c' is the source file
# '-o oop_demo' specifies the name of the output executable
# '-std=c99' (Optional) ensures it compiles with the C99 standard
gcc oop_in_c.c -o oop_demo -std=c99

./oop_demo

You will see a log demonstrating the creation of "objects," the polymorphic calls to attack(), and the final "virtual" cleanup process.

--- 1. CREATING "OBJECTS" ---
[Entity 'Grom' initialized]
  -> Warrior 'Grom' created!
[Entity 'Jaina' initialized]
  -> Mage 'Jaina' created!

--- 2. DEMONSTRATING POLYMORPHISM ---
Calling attack() for: Grom
Grom swings their axe! (Rage: 50)
Calling attack() for: Jaina
Jaina casts Fireball! (Mana: 100)

--- 3. DEMONSTRATING ENCAPSULATION ---
Calling 'public' method Entity_takeDamage() on Grom.
Grom takes 30 damage.
Displaying updated status:
== Warrior == | Name: Grom | Health: 120 | Rage: 60

--- 4. POLYMORPHIC CLEANUP ---
  -> Warrior 'Grom' is being destroyed.
[Entity 'Grom' cleaned up]
  -> Mage 'Jaina' is being destroyed.
[Entity 'Jaina' cleaned up]