Emulating Classes in C using Structs

Question

I am constrained to using C for a competition and I have a need to emulate classes. I am trying to construct a simple "point" class that can return and set the X and Y coordinates of a point. Yet, the below code returns errors such as "unknown type name point", "expected identifier or (" and "expected parameter declarator." What do these errors mean? How do I correct them? Is this the correct approach to writing a "pseudo-class"?

typedef struct object object, *setCoordinates;

struct object {
    float x, y;
    void (*setCoordinates)(object *self, float x, float y);
    void (*getYCoordinate)(object *self);
    void (*getXCoordinate)(object *self);
};

void object_setCoordinates(object *self, float x, float y){
    self->x = x;
    self->y = y;
}

float object_getXCoordinate(object *self){
    return self->x;
}

float object_getYCoordinate(object *self){
    return self->y;
}

object point;
point.setCoordinates = object_setCoordinates;
point.getYCoordinate = object_getYCoordinate;
point.getXCoordinate = object_getXCoordinate;

point.setCoordinates(&point, 1, 2);
printf("Coordinates: X Coordinate: %f, Y Coordinate: %f", point.getXCoordinate, point.getYCoordinate);

Reference: 1. C - function inside struct 2. How do you implement a class in C?

Answer 1

You would do much better to implement it as follows:

#include <stdio.h>

struct point {
    float x;
    float y;
};

void point_setCoordinates(struct point *self, float x, float y){
    self->x = x;
    self->y = y;
}

float point_getXCoordinate(struct point *self){
    return self->x;
}

float point_getYCoordinate(struct point *self){
    return self->y;
}

int main(void) {
    struct point my_point;

    point_setCoordinates(&my_point, 1, 2);

    printf("Coordinates: X Coordinate: %f, Y Coordinate: %f\n",
           point_getXCoordinate(&my_point),
           point_getYCoordinate(&my_point));

    return 0;
}

A few things to note:

• As @Olaf has pointed out, never typedef a pointer - it hides your intent and makes things unclear. Yes, it's all over poor APIs (e.g: Windows), but it reduces readability.
• You really don't need these functions to be the equivalent to virtual functions... just have a set of point_*() functions that you call on the point 'thing'.
• Don't confuse things with poor names... if it's an X,Y point, then call it such - not an object (which is a very generic concept).
• You need to call functions... in your call to printf() you used point.getXCoordinate - that is to say you took it's address and asked printf() to display it as though it were a float
• You might start to wonder why you'd care about calling a function to get access to a variable that is inside a transparent struct... See below.

---

Many libraries / APIs provide opaque datatypes. This means that you can get a 'handle' to a 'thing'... but you have no idea what's being stored within the 'thing'. The library then provides you with access functions, as shown below. This is how I'd advise you approach the situation.

Don't forget to free the memory!

I've implemented an example below.

point.h

#ifndef POINT_H
#define POINT_H

struct point;

struct point *point_alloc(void);
void point_free(struct point *self);

void point_setCoordinates(struct point *self, float x, float y);
float point_getXCoordinate(struct point *self);
float point_getYCoordinate(struct point *self);

#endif /* POINT_H */

point.c

#include <stdlib.h>
#include <string.h>

#include "point.h"

struct point {
    float x;
    float y;
};

struct point *point_alloc(void) {
    struct point *point;

    point = malloc(sizeof(*point));
    if (point == NULL) {
        return NULL;
    }

    memset(point, 0, sizeof(*point));

    return point;
}

void point_setCoordinates(struct point *self, float x, float y) {
    self->x = x;
    self->y = y;
}

float point_getXCoordinate(struct point *self) {
    return self->x;
}

float point_getYCoordinate(struct point *self) {
    return self->y;
}

void point_free(struct point *self) {
    free(self);
}

main.c

#include <stdio.h>

#include "point.h"

int main(void) {
    struct point *point;

    point = point_alloc();

    point_setCoordinates(point, 1, 2);

    printf("Coordinates: X Coordinate: %f, Y Coordinate: %f\n",
           point_getXCoordinate(point),
           point_getYCoordinate(point));

    point_free(point);

    return 0;
}

Answer 2

Your code has some minor errors. That's why it doesn't compile.

Fixed here:

typedef struct object object;

struct object {
    float x, y;
    void (*setCoordinates)(object *self, float x, float y);
    float (*getYCoordinate)(object *self);
    float (*getXCoordinate)(object *self);
};

void object_setCoordinates(object *self, float x, float y){
    self->x = x;
    self->y = y;
}

float object_getXCoordinate(object *self){
    return self->x;
}

float object_getYCoordinate(object *self){
    return self->y;
}

int main()
{

    object point;
    point.setCoordinates = object_setCoordinates;
    point.getYCoordinate = object_getYCoordinate;
    point.getXCoordinate = object_getXCoordinate;

    point.setCoordinates(&point, 1, 2);
    printf("Coordinates: X Coordinate: %f, Y Coordinate: %f", 
    point.getXCoordinate(&point), point.getYCoordinate(&point));
}

As for the approach, there's probably no need to store the pointers to your methods inside the struct when you can simply call them directly:

object x;
object_setCoordinates(x, 1, 2);
//...

Answer 3

I also have an example of basic class emulation in C [the OP specified for a specific application, although, this answer is to the general question]:

A header file called "c_class.h"

#ifndef CLASS_HEADER_H
#define CLASS_HEADER_H

// Function pointer prototypes used by these classes
typedef int sub_func_t (int);
typedef float sub_funcf_t (int,int);

/* class type definition 
  (emulated class type definition; C doesn't really have class types) */
typedef struct {
    //Data Variables
    int a;

    /*Function (also known as Method) pointers
      (note that different functions have the same function pointer prototype)*/
    sub_func_t* add;
    sub_func_t* subt;
    sub_func_t* mult;
    sub_funcf_t* div;  
} class_name;


// class init prototypes
// These inits connect the function pointers to specific functions
// and initialize the variables.
class_name* class_init_ptr (int, sub_func_t*, sub_func_t*, sub_func_t*, sub_funcf_t*);
class_name class_init (int, sub_func_t*, sub_func_t*, sub_func_t*, sub_funcf_t*);

#endif

A source code file called "c_class.c"

//gcc -o c_class c_class.c

#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
#include"c_class.h"

// The class function definitions.

/*
    If we make these member functions static then they are only 
    accessible via code from this file.
    However, we can still pass the class-like objects around a 
    larger program and access their member functions,
    just like in any OO language.
    
    It is possible to emulate inheritance by declaring a class object 
    from the class type definition (I don't touch on these more 
    abstract subjects though, this is only a basic class emulation).
*/
    
static int AddFunc(int num){
    num++;
    return num;
}

static int SubtFunc(int num){
    num--;
    return num;  
}

static int MultFunc(int num){
    num *= num;
    return num;
}

static float DivFunc(int num, int denom){
    float fnum = (float)num / (float)denom;
    return fnum;  
}

// The class init function definitions.
class_name* class_init_ptr (int num, sub_func_t* addition, sub_func_t* subtraction, sub_func_t* multiplication, sub_funcf_t* division) 
{ 
    class_name* new_class = malloc(sizeof(*new_class)); 
    assert(new_class != NULL);
    *new_class = (class_name){num, addition, subtraction, multiplication, division};
    /*We could also just type:
    new_class->a = num;
    new_class->add = addition;
    new_class->subt = subtraction;   
    new_class->mult = multiplication; 
    new_class->div = division;  
    */
    return new_class; 
}

class_name class_init(int num, sub_func_t* addition, sub_func_t* subtraction, sub_func_t* multiplication, sub_funcf_t* division) 
{ 
    class_name new_class; 
    new_class = (class_name){num, addition, subtraction, multiplication, division};
    /* We could also just type:
    new_class.a = num;
    new_class.add = addition;
    new_class.subt = subtraction;   
    new_class.mult = multiplication; 
    new_class.div = division;  
    */
    return new_class; 
}

//Working Function Prototypes
class_name* Working_Function(class_name*);
class_name Working_Function_Two(class_name);

int main(){
    /* It's possible to connect the functions within the init also,
       w/o sending them. */
    class_name *MyClass = class_init_ptr(5, AddFunc, SubtFunc, MultFunc, DivFunc);
    class_name MyOtherClass = class_init(0, AddFunc, SubtFunc, MultFunc, DivFunc);
    
    printf("%i\n",MyClass->add(100));// 101
    
    printf("%i\n",MyClass->subt(100));// 99

    printf("%i\n",MyClass->mult(100));// 10000

    printf("%f\n",MyClass->div(MyClass->a,2)); // 2.5
    
    printf("%i\n",MyClass->mult(MyClass->mult(100))); //100000000

    MyClass = Working_Function(MyClass);
    //This would work also (because we're passing a pointer):
    //Working_Function(MyClass); 
   printf("%i\n",MyClass->a); //a = 5000

    MyOtherClass = Working_Function_Two(MyOtherClass);
    printf("%i\n",MyOtherClass.a); //a = 9999

    MyOtherClass.a = 25;
    Working_Function_Two(MyOtherClass); //pass by value
    printf("%i\n",MyOtherClass.a); //a = 25  (no value change)

    Working_Function(&MyOtherClass); //pass by reference
    printf("%i\n",MyOtherClass.a); //a = 5000 (value changed)

    return 0;
}

//Working Functions
class_name* Working_Function(class_name* PassedClass){
    printf("%i\n",PassedClass->a);// 5, then 25
    printf("%i\n",PassedClass->add(PassedClass->a));// 6, then 26
    PassedClass->a = 5000;
    return PassedClass;
}

class_name Working_Function_Two(class_name PassedClass){
    printf("%i\n",PassedClass.a);// 0, then 25
    printf("%i\n",PassedClass.add(PassedClass.a));// 1, then 26
    PassedClass.a = 9999;
    return PassedClass;
}

/* We're passing emulated class objects and emulated class pointers 
   by reference and value, if everything works it should print this:

101
99
10000
2.500000
100000000
5
6
5000
0
1
9999
25
26
25
25
26
5000

*/

Answer 4

Another way to write a pseudo-class that needs polymorphism, with less overhead per instance, is to create a single virtual function table and have your constructor or factory function set that. Here’s a hypothetical example. (Edit: Now a MCVE, but for real code, refactor into header and separate source files.)

#include <assert.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

struct point; // Abstract base class.

struct point_vtable {
  void (*setCoordinates)(struct point *self, float x, float y);
  float (*getYCoordinate)(const struct point *self);
  float (*getXCoordinate)(const struct point *self);
};

typedef struct point {
  const struct point_vtable* vtable;
} point;

typedef struct cartesian_point {
  const struct point_vtable* vtable;
  float x;
  float y;
} cartesian_point;

typedef struct polar_point {
  const struct point_vtable* vtable;
  float r;
  float theta;
} polar_point;

void cartesian_setCoordinates( struct point* self, float x, float y );
float cartesian_getXCoordinate(const struct point* self);
float cartesian_getYCoordinate(const struct point* self);

void polar_setCoordinates( struct point* self, float x, float y );
float polar_getXCoordinate(const struct point* self);
float polar_getYCoordinate(const struct point* self);

const struct point_vtable cartesian_vtable = {
  .setCoordinates = &cartesian_setCoordinates,
  .getXCoordinate = &cartesian_getXCoordinate,
  .getYCoordinate = &cartesian_getYCoordinate
};

const struct point_vtable polar_vtable = {
  .setCoordinates = &polar_setCoordinates,
  .getXCoordinate = &polar_getXCoordinate,
  .getYCoordinate = &polar_getYCoordinate
};

void cartesian_setCoordinates( struct point* const self,
                               const float x,
                               const float y )
{
  assert(self->vtable == &cartesian_vtable);
  struct cartesian_point * const this = (struct cartesian_point*)self;
  this->x = x;
  this->y = y;
}

float cartesian_getXCoordinate(const struct point* const self)
{
  assert(self->vtable == &cartesian_vtable);
  const struct cartesian_point * const this = (struct cartesian_point*)self;
  return this->x;
}

float cartesian_getYCoordinate(const struct point* const self)
{
  assert(self->vtable == &cartesian_vtable);
  const struct cartesian_point * const this = (struct cartesian_point*)self;
  return this->y;
}

void polar_setCoordinates( struct point* const self,
                           const float x,
                           const float y )
{
  assert(self->vtable == &polar_vtable);
  struct polar_point * const this = (struct polar_point*)self;
  this->theta = (float)atan2((double)y, (double)x);
  this->r = (float)sqrt((double)x*x + (double)y*y);
}

float polar_getXCoordinate(const struct point* const self)
{
  assert(self->vtable == &polar_vtable);
  const struct polar_point * const this = (struct polar_point*)self;
  return (float)((double)this->r * cos((double)this->theta));
}

float polar_getYCoordinate(const struct point* const self)
{
  assert(self->vtable == &polar_vtable);
  const struct polar_point * const this = (struct polar_point*)self;
  return (float)((double)this->r * sin((double)this->theta));
}

// Suitable for the right-hand side of initializations, before the semicolon.
#define CARTESIAN_POINT_INITIALIZER { .vtable = &cartesian_vtable,\
                                      .x = 0.0F, .y = 0.0F }
#define POLAR_POINT_INITIALIZER { .vtable = &polar_vtable,\
                                  .r = 0.0F, .theta = 0.0F }

int main(void)
{
  polar_point another_point = POLAR_POINT_INITIALIZER;
  point* const p = (point*)&another_point; // Base class pointer.
  polar_setCoordinates( p, 0.5F, 0.5F ); // Static binding.
  const float x = p->vtable->getXCoordinate(p); // Dynamic binding.
  const float y = p->vtable->getYCoordinate(p); // Dynamic binding.

  printf( "(%f, %f)\n", x, y );
  return EXIT_SUCCESS;  
}

This takes advantage of the guarantee that the common initial subsequence of structs can be addressed through a pointer to any of them, and stores only one pointer of class overhead per instance, not one function pointer per virtual function. You can use the virtual table as your class identifier for your variant structure. Also, the virtual table cannot contain garbage. Virtual function calls need to dereference two pointers rather than one, but the virtual table of any class in use is highly likely to be in the cache.

I also note that this interface is very skeletal; it’s silly to have a polar class that can do nothing but convert back to Cartesian coordinates, and any implementation like this would at minimum need some way to initialize dynamic memory.

If you don’t need polymorphism, see Attie’s much simpler answer.