How do you implement a class in C? [closed]

Answer 1

Also see this answer and this one

It is possible. It always seems like a good idea at the time but afterwards it becomes a maintenance nightmare. Your code become littered with pieces of code tying everything together. A new programmer will have lots of problems reading and understanding the code if you use function pointers since it will not be obvious what functions is called.

Data hiding with get/set functions is easy to implement in C but stop there. I have seen multiple attempts at this in the embedded environment and in the end it is always a maintenance problem.

Since you all ready have maintenance issues I would steer clear.

Answer 2

I will give a simple example of how OOP should be done in C. I realise this thead is from 2009 but would like to add this anyway.

/// Object.h
typedef struct Object {
    uuid_t uuid;
} Object;

int Object_init(Object *self);
uuid_t Object_get_uuid(Object *self);
int Object_clean(Object *self);

/// Person.h
typedef struct Person {
    Object obj;
    char *name;
} Person;

int Person_init(Person *self, char *name);
int Person_greet(Person *self);
int Person_clean(Person *self);

/// Object.c
#include "object.h"

int Object_init(Object *self)
{
    self->uuid = uuid_new();

    return 0;
}
uuid_t Object_get_uuid(Object *self)
{ // Don't actually create getters in C...
    return self->uuid;
}
int Object_clean(Object *self)
{
    uuid_free(self->uuid);

    return 0;
}

/// Person.c
#include "person.h"

int Person_init(Person *self, char *name)
{
    Object_init(&self->obj); // Or just Object_init(&self);
    self->name = strdup(name);

    return 0;
}
int Person_greet(Person *self)
{
    printf("Hello, %s", self->name);

    return 0;
}
int Person_clean(Person *self)
{
    free(self->name);
    Object_clean(self);

    return 0;
}

/// main.c
int main(void)
{
    Person p;

    Person_init(&p, "John");
    Person_greet(&p);
    Object_get_uuid(&p); // Inherited function
    Person_clean(&p);

    return 0;
}

The basic concept involves placing the 'inherited class' at the top of the struct. This way, accessing the first 4 bytes in the struct also accesses the first 4 bytes in the 'inherited class' (Asuming non-crazy optimalisations). Now, when the pointer of the struct is cast to the 'inherited class', the 'inherited class' can access the 'inherited values' in the same way it would access it's members normally.

This and some naming conventions for constructors, destructors, allocation and deallocarion functions (I recommend init, clean, new, free) will get you a long way.

As for Virtual functions, use function pointers in the struct, possibly with Class_func(...); wrapper too. As for (simple) templates, add a size_t parameter to determine size, require a void* pointer, or require a 'class' type with just the functionality you care about. (e.g. int GetUUID(Object *self); GetUUID(&p);)

Answer 3

In your case the good approximation of the class could be the an ADT. But still it won't be the same.

Answer 4

C isn't an OOP language, as your rightly point out, so there's no built-in way to write a true class. You're best bet is to look at structs, and function pointers, these will let you build an approximation of a class. However, as C is procedural you might want to consider writing more C-like code (i.e. without trying to use classes).

Also, if you can use C, you can probally use C++ and get classes.

Answer 5

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.
/// 

Answer 6

GTK is built entirely on C and it uses many OOP concepts. I have read through the source code of GTK and it is pretty impressive, and definitely easier to read. The basic concept is that each "class" is simply a struct, and associated static functions. The static functions all accept the "instance" struct as a parameter, do whatever then need, and return results if necessary. For Example, you may have a function "GetPosition(CircleStruct obj)". The function would simply dig through the struct, extract the position numbers, probably build a new PositionStruct object, stick the x and y in the new PositionStruct, and return it. GTK even implements inheritance this way by embedding structs inside structs. pretty clever.