Object Oriented Programming in C

If there is C++, what is the need of doing Object Oriented Programming (OOP) in C? Well I don’t have a compelling reason other than that it is a fun exercise.  So, with that in mind, I am going to show an approach to OOP in C next. It is as close to OOP but not exactly OOP - for one it is not compiler restricted programming - that is compiler won’t crib if I break the OOP rules - and two, this implementation is short on some parameters of OOP. Which I will examine as I go along the example.

Note: The programmer  should understand the underlying assumptions and should enforce the rules of programming to achieve OOP in C, as outlined below.

I will use two main features of C to attain encapsulation and function polymorphism. Namely, function pointers and static functions. Accompanied by segregation of public and private members in header files.

I start with a public header file –publicclass.h- of the class, which should be included wherever that class is needed.

typedef struct _testClass testClass; struct _testClass {     int publicData;         void (*publicFunc)(testClass*); }; // function to create testClass testClass *constructClass(void); void destructClass(testClass*);

The structure testClass  above defines public members of the class. Every function declared has to take a pointer to the structure of which it is a member. This  is done to emulate 'this', as in C++. So, that we know the object being referred to. Also constructor and a destructor functions are defined, which create and destroy the class object respectively. These are like C++ constructors/destructors in a loose sense. These have to be called the first (for constructor) and the last (for destructor) in the object life cycle. The public function pointer defined will be discussed later.

Next I declare a private header file –privateclass.h- , to encapsulate the private data of the class. This is done by exclusively including this file in the class declaration file – class.c- and not anywhere else. This is one of the user coding restrictions to enforce data encapsulation.

#include "publicclass.h" typedef struct _itestClass itestClass; struct _itestClass { // public:     testClass public; // private:     int privateData;         void (*privateFunc)(itestClass*); };

The structure above is called itestClass, where ‘i’ stands for internal or more appropriately private (I avoided using ‘p’ for private as to not confuse it with pointer).  This structure looks similar to the previous one but with one difference, that is, it includes the public ‘testClass’ structure on the top - it is the first member. This serves a simple purpose of simplifying the pointer arithmetic, such that the pointer to ‘public’ (of ‘testClass’) will be the pointer to ‘itestClass’. This will become clear on the actual class implementation, below. Note, that the private functions takes pointer of ‘itestClass’.

Finally, the class implementation –class.c-. It is more or less self explanatory, where both public and private functions are defined. And in constructor assigned to the created object, and the pointer to the public structure –testClass- returned.

#include "publicclass.h" #include "privateclass.h" static void publicFunc(testClass *this) {     itestClass *ithis = (itestClass*)this;         printf("This is publicFunc\n");     this->publicData = 20;     printf("From publicFunc: publicData: %d\n", this->publicData);         ithis->privateData = 30;     printf("From publicFunc: privateData: %d\n\n", ithis->privateData);         ithis->privateFunc(ithis); } static void privateFunc(itestClass *ithis) {     printf("This is privateFunc\n"); } testClass *constructClass(void) {     itestClass *pobj = (itestClass*)calloc(1, sizeof(itestClass));         pobj->public.publicData = 0;     pobj->public.publicFunc = publicFunc;     pobj->privateData = 0;     pobj->privateFunc = privateFunc;     return &pobj->public; } void destructClass(testClass *pobj){     if (pobj) { free(pobj); } }

As mentioned earlier ‘this’ (pointer to class object) is used to get ‘ithis’ (pointer to internal class object). In a class declaration all the functions are ‘static’ (except the constructor and destructor), to avoid namespace pollution and achieve function polymorphism. Such that, another class can be defined with a different implementation of the functions. Apart from function polymorphism class hierarchy can also be achieved and that is left as an exercise for the reader :-).  Note: only this file includes the private header file –privateclass.h.

And lastly here is how to use the above built OOP structure in C.

#include "publicclass.h" int main (int argc, const char * argv[]) {         testClass *pobj = constructClass();     pobj->publicData = 10;     printf("From main: publicData: %d\n\n", pobj->publicData);         // Error: not a member of obj(testClass) and itestClass is not visible     // pobj->privateData = 20;         pobj->publicFunc(pobj);         destructClass(pobj);     return 0; }

Output.

From main: publicData: 10 This is publicFunc From publicFunc: publicData: 20 From publicFunc: privateData: 30 This is privateFunc

 Download this example from here.

(I am no longer using line numbers to post the code. I realized that with line numbers it is difficult to make changes in the post. On a change I have to update line numbers, else leave it discontinuous.)