How Do you implements class in C ?

Assuming I have to use C (no C++ or object oriented compilers) and I don't have dynamic memory allocation, what are some techniques I can use to implement a class, or a good approximation of a class? Is it always a good idea to isolate the "class" to a separate file? Assume that we can preallocate the memory by assuming a fixed number of instances, or even defining the reference to each object as a constant before compile time. Feel free to make assumptions about which OOP concept I will need to implement (it will vary) and suggest the best method for each.

Restrictions:

* I have to use C and not an OOP because I'm writing code for an embedded system, and the compiler and preexisting code base is in C.
* There is no dynamic memory allocation because we don't have enough memory to reasonably assume we won't run out if we start dynamically allocating it.
* The compilers we work with have no problems with function pointers


Answer:

That depends on the exact "object-oriented" feature-set you want to have. If you need stuff like overloading and/or virtual methods, you probably need to include function pointers in structures:

typedef struct {
float (*computeArea)(const ShapeClass *shape);
} ShapeClass;

float shape_computeArea(const ShapeClass *shape)
{
return shape->computeArea(shape);
}

This would let you implement a class, by "inheriting" the base class, and implementing a suitable function:

typedef struct {
ShapeClass shape;
float width, height;
} RectangleClass;

static float rectangle_computeArea(const ShapeClass *shape)
{
const RectangleClass *rect = (const RectangleClass *) shape;
return rect->width * width->height;
}

This of course requires you to also implement a constructor, that makes sure the function pointer is properly set up. Normally you'd dynamically allocate memory for the instance, but you can let the caller do that, too:

void rectangle_new(RectangleClass *rect)
{
rect->width = rect->height = 0.f;
rect->shape.computeArea = rectangle_computeArea;
}

If you want several different constructors, you will have to "decorate" the function names, you can't have more than one rectangle_new() function:

void rectangle_new_with_lengths(RectangleClass *rect, float width, float height)
{
rectangle_new(rect);
rect->width = width;
rect->height = height;
}

Here's a basic example showing usage:

int main(void)
{
RectangleClass r1;

rectangle_new_with_lenghts(&r1, 4.f, 5.f);
printf("rectangle r1's area is %f units square\n", shape_computeArea(&r1));
return 0;
}

I hope this gives you some ideas, at least. For a successful and rich object-oriented framework in C