OOP คือ C — Object-Oriented Programming ในภาษา C

OOP ในภาษา C

OOP คือ C Object-Oriented Programming Encapsulation Inheritance Polymorphism Abstraction struct function pointer opaque vtable

OOP Concept	C Implementation	C++ Equivalent
Class	struct + functions	class
Encapsulation	opaque pointer + getter/setter	private/public
Inheritance	struct embedding + cast	class : public Base
Polymorphism	function pointer in struct	virtual function
Constructor	create() function + malloc	constructor
Destructor	destroy() function + free	destructor

Encapsulation

/* === Encapsulation in C === */

/* person.h - Public Interface */
/* typedef struct Person Person;  // Opaque - ไม่เห็น member */
/* Person* person_create(const char* name, int age); */
/* void person_destroy(Person* p); */
/* const char* person_get_name(const Person* p); */
/* int person_get_age(const Person* p); */
/* void person_set_age(Person* p, int age); */

/* person.c - Private Implementation */
/* #include "person.h" */
/* #include <stdlib.h> */
/* #include <string.h> */
/*  */
/* struct Person {           // Private - เห็นเฉพาะใน .c */
/*     char name[100]; */
/*     int age; */
/* }; */
/*  */
/* Person* person_create(const char* name, int age) { */
/*     Person* p = malloc(sizeof(Person)); */
/*     strncpy(p->name, name, 99); */
/*     p->age = age; */
/*     return p; */
/* } */
/*  */
/* void person_destroy(Person* p) { free(p); } */
/* const char* person_get_name(const Person* p) { return p->name; } */
/* int person_get_age(const Person* p) { return p->age; } */
/* void person_set_age(Person* p, int age) { */
/*     if (age > 0 && age < 200) p->age = age;  // Validation */
/* } */

/* Usage: */
/* Person* p = person_create("สมชาย", 25); */
/* printf("Name: %s Age: %d\n", person_get_name(p), person_get_age(p)); */
/* person_set_age(p, 26); */
/* person_destroy(p); */

from dataclasses import dataclass

@dataclass
class OOPPattern:
    concept: str
    c_technique: str
    header_file: str
    source_file: str

patterns = [
    OOPPattern("Encapsulation",
        "Opaque Pointer (typedef struct X X;)",
        "ประกาศ type + function prototype เท่านั้น",
        "define struct member + implement function"),
    OOPPattern("Private Members",
        "struct member อยู่ใน .c file เท่านั้น",
        "ผู้ใช้ไม่เห็น member ใน .h",
        "เข้าถึง member ได้เฉพาะใน .c"),
    OOPPattern("Getter/Setter",
        "function รับ pointer + return/set value",
        "const char* get_name(const X* x);",
        "return x->name; x->age = age;"),
    OOPPattern("Constructor",
        "create() function + malloc + init",
        "X* x_create(params);",
        "malloc + set member + return pointer"),
    OOPPattern("Destructor",
        "destroy() function + free",
        "void x_destroy(X* x);",
        "free(x); // cleanup resources"),
]

print("=== Encapsulation Patterns ===")
for p in patterns:
    print(f"  [{p.concept}] {p.c_technique}")
    print(f"    .h: {p.header_file}")
    print(f"    .c: {p.source_file}")

Inheritance & Polymorphism

/* === Inheritance & Polymorphism in C === */

/* Base: Shape */
/* typedef struct Shape { */
/*     int x, y; */
/*     double (*area)(void* self);     // Virtual function */
/*     void (*draw)(void* self);       // Virtual function */
/*     void (*destroy)(void* self);    // Virtual destructor */
/* } Shape; */

/* Derived: Circle */
/* typedef struct { */
/*     Shape base;       // MUST be first member (for casting) */
/*     int radius; */
/* } Circle; */
/*  */
/* double circle_area(void* self) { */
/*     Circle* c = (Circle*)self; */
/*     return 3.14159 * c->radius * c->radius; */
/* } */
/*  */
/* Circle* circle_create(int x, int y, int r) { */
/*     Circle* c = malloc(sizeof(Circle)); */
/*     c->base.x = x; */
/*     c->base.y = y; */
/*     c->base.area = circle_area;       // Set virtual function */
/*     c->base.draw = circle_draw; */
/*     c->base.destroy = circle_destroy; */
/*     c->radius = r; */
/*     return c; */
/* } */

/* Derived: Rectangle */
/* typedef struct { */
/*     Shape base; */
/*     int width, height; */
/* } Rectangle; */
/*  */
/* double rect_area(void* self) { */
/*     Rectangle* r = (Rectangle*)self; */
/*     return r->width * r->height; */
/* } */

/* Polymorphism: */
/* Shape* shapes[3]; */
/* shapes[0] = (Shape*)circle_create(0, 0, 5); */
/* shapes[1] = (Shape*)rect_create(0, 0, 10, 20); */
/* shapes[2] = (Shape*)circle_create(5, 5, 3); */
/*  */
/* for (int i = 0; i < 3; i++) { */
/*     printf("Area: %.2f\n", shapes[i]->area(shapes[i])); */
/*     shapes[i]->destroy(shapes[i]);  // Virtual destructor */
/* } */

@dataclass
class InheritancePattern:
    pattern: str
    c_code: str
    cpp_equivalent: str
    note: str

inheritance = [
    InheritancePattern("Base Class",
        "typedef struct { int x; double (*method)(void*); } Base;",
        "class Base { virtual double method() = 0; };",
        "function pointer = virtual function"),
    InheritancePattern("Derived Class",
        "typedef struct { Base base; int extra; } Derived;",
        "class Derived : public Base { int extra; };",
        "base ต้องเป็น member ตัวแรก"),
    InheritancePattern("Upcasting",
        "Base* b = (Base*)&derived;",
        "Base* b = &derived; // Implicit",
        "C ต้อง cast explicit C++ implicit"),
    InheritancePattern("Virtual Call",
        "b->method(b); // Call through function pointer",
        "b->method(); // Virtual dispatch",
        "C ส่ง self เอง C++ ส่ง this อัตโนมัติ"),
    InheritancePattern("Virtual Destructor",
        "b->destroy(b); // function pointer",
        "virtual ~Base() {}; delete b;",
        "C ต้องเรียก destroy เอง C++ อัตโนมัติ"),
]

print("=== Inheritance Patterns ===")
for i in inheritance:
    print(f"  [{i.pattern}]")
    print(f"    C: {i.c_code}")
    print(f"    C++: {i.cpp_equivalent}")
    print(f"    Note: {i.note}")

Real-world Examples

# === Real-world OOP in C ===

@dataclass
class RealExample:
    project: str
    oop_usage: str
    technique: str
    example: str

examples = [
    RealExample("Linux Kernel VFS",
        "Virtual File System ใช้ OOP Pattern",
        "struct file_operations มี function pointer (read write open close)",
        "ext4 xfs btrfs implement file_operations ต่างกัน → Polymorphism"),
    RealExample("GTK+ / GObject",
        "GUI Framework เขียนด้วย C แบบ OOP เต็มรูปแบบ",
        "GObject System: Inheritance Encapsulation Polymorphism Signal",
        "GtkWidget → GtkContainer → GtkBox → GtkButton (Inheritance chain)"),
    RealExample("SQLite",
        "Database Engine ใช้ OOP Pattern สำหรับ VFS Backend",
        "sqlite3_vfs struct มี function pointer สำหรับ File I/O",
        "Custom VFS สำหรับ In-memory Encrypted Compressed Storage"),
    RealExample("CPython",
        "Python Interpreter เขียนด้วย C ใช้ OOP",
        "PyObject base struct ทุก Python Object สืบทอดจาก PyObject",
        "PyLongObject PyListObject PyDictObject ล้วนมี PyObject เป็น base"),
    RealExample("Embedded Device Driver",
        "Device Driver ใช้ OOP Pattern สำหรับ HAL",
        "struct device_ops { init read write close } แต่ละ device implement ต่างกัน",
        "UART SPI I2C Driver ใช้ struct เดียวกัน implement ต่างกัน"),
]

print("=== Real-world OOP in C ===")
for e in examples:
    print(f"\n  [{e.project}] {e.oop_usage}")
    print(f"    Technique: {e.technique}")
    print(f"    Example: {e.example}")

เคล็ดลับ

• Opaque: ใช้ Opaque Pointer ซ่อน struct member ใน .c file
• First Member: Base struct ต้องเป็น member ตัวแรกเสมอ สำหรับ Cast
• Self: ส่ง self pointer เป็น parameter ตัวแรกของทุก method
• Naming: ใช้ prefix ชื่อ struct ทุก function เช่น person_create circle_area
• Memory: อย่าลืม destroy/free ทุก object ที่ create C ไม่มี GC

OOP คืออะไร

Object-Oriented Programming Encapsulation Inheritance Polymorphism Abstraction struct function pointer C จำลอง OOP Linux GObject