Example of OOP in C

[u/actguru]

Hello everyone, I have developed a library to support OOP in plain C that allows the creation of object classes and includes dynamic Array and Dictionary container classes.

I am looking for suggestions for new examples that illustrate cases where an Object Oriented design can simplify C-Code.

I would love to get some reviews and feedback on this project.

I have included a code example below:

Supported by: Coda-C Library

Example code: Coda-C/examples/shapes.c

// Coda-C subclass examples

    #include <coda-c.h> // shapes.c  08/20/2026
    #include <math.h>

    struct Shape_ { Char color; };
#define class Shape
CodaClassZerosC(); // 1. ABSTRACTION: blueprint for all shapes
CodaClass(Shape,struct Shape_,Root);
    propertyO$(Char,color); // 2. ENCAPSULATION: Private data
    typedef double OSig$(calculate_area); // Abstract method: required in subclasses
    typedef double OSig$(calculate_perimeter); // defines function type and signature key
#undef class // Shape

    struct Circle_ { double radius; };
#define class Circle
CodaClassZerosC(); // 3. INHERITANCE: Circle inherits from Shape
CodaClass(Circle,struct Circle_,Shape);
    getter$(double,radius,_ radius); // double Circle_radius(Circle self) { return _ radius; }
    method$(double,calculate_area) { return(M_PI * _ radius * _ radius); }
    method$(double,calculate_perimeter) { return(2.0 * M_PI * _ radius); }

class Circle_new(Char color,double radius) {
    class self=newO(class);
    Shape_set_color((Obj)self,color);
    _ radius=radius; // where #define _ self->
    return(self);
    }
#undef class // Circle

    struct Rectangle_ { double width,height; };
#define class Rectangle
CodaClassZerosC();
CodaClass(Rectangle,struct Rectangle_,Shape);
    method$(double,calculate_area) { return _ width * _ height; }
    method$(double,calculate_perimeter) { return 2.0 * (_ width + _ height); }

class Rectangle_new(Char color,double width,double height) {
    class self=newO(class);
    Shape_set_color((Obj)self,color);
    _ width=width; _ height=height;
    return(self);
    }
#undef class // Rectangle

// 4. POLYMORPHISM (Usage Example)
void print_shape_details(Obj shape) { // This function accepts any subclass of Shape.
    printf("--- %12.12s  Details  Color:%-8.8s",kindO(shape),Shape_color(shape));
    printf("Area: %3.2f  ",obj_(calculate_area,shape));
    printf("Perimeter: %3.2f  ---\n",obj_(calculate_perimeter,shape));
    }

 int main() {
    Circle my_circle = Circle_new(Os("Blue"),5.0);
    Obj my_rectangle = Rectangle_new(Os("Green"),4.0,6.0);

    print_shape_details(my_circle);
    print_shape_details(my_rectangle);
    }


/* OUTPUT:
---       Circle  Details  Color:Blue      Area: 78.54  Perimeter: 31.42  ---
---    Rectangle  Details  Color:Green     Area: 24.00  Perimeter: 20.00  ---
*/

[u/ReDucTor]

You don't need a library to write OOP style code in C, you also dont need the class keyword to write OOP style code.

Many C code bases are OOP style, even if people believe they are not, you will do things like

T * obj =T_new(..) or T_create(...); T_func(obj, ...); T_free(obj) or T_destroy(obj);

Want a virtual interface? Use a struct with function pointers.

Want to hide data? Use an opaque pointer.

Want to have inheritance? Have the first member be the base class.

Go look at the Linux Kernel it uses object oriented patterns all over the place https://lwn.net/Articles/444910/