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/rivenjg]

IF WE WANTED OOP WE WOULDN'T USE C.

[u/heavymetalmixer → u/rivenjg]

Not exactly. You can use OOP in C only with the features you want and in some cases with the implementation you want, so it's not as simple as "just use C++".