How do you implement a class in C?

I had to do it once too for a homework. I followed this approach:

1. Define your data members in a struct.
2. Define your function members that take a pointer to your struct as first argument.
3. Do these in one header & one c. Header for struct definition & function declarations, c for implementations.

A simple example would be this:

/// Queue.h
struct Queue
{
    /// members
}
typedef struct Queue Queue;

void push(Queue* q, int element);
void pop(Queue* q);
// etc.
/// 

If you only want one class, use an array of structs as the "objects" data and pass pointers to them to the "member" functions. You can use typedef struct _whatever Whatever before declaring struct _whatever to hide the implementation from client code. There's no difference between such an "object" and the C standard library FILE object.

If you want more than one class with inheritance and virtual functions, then it's common to have pointers to the functions as members of the struct, or a shared pointer to a table of virtual functions. The GObject library uses both this and the typedef trick, and is widely used.

There's also a book on techniques for this available online - Object Oriented Programming with ANSI C.

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 * rect->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_lengths(&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, look into glib's GObject library.

Also note that there's no explicit "class" being modelled above, each object has its own method pointers which is a bit more flexible than you'd typically find in C++. Also, it costs memory. You could get away from that by stuffing the method pointers in a class structure, and invent a way for each object instance to reference a class.

C Interfaces and Implementations: Techniques for Creating Reusable Software, David R. Hanson

http://www.informit.com/store/product.aspx?isbn=0201498413

This book does an excellent job of covering your question. It's in the Addison Wesley Professional Computing series.

The basic paradigm is something like this:

/* for data structure foo */

FOO *myfoo;
myfoo = foo_create(...);
foo_something(myfoo, ...);
myfoo = foo_append(myfoo, ...);
foo_delete(myfoo);