Object oriented programming in C [duplicate]

Answer 1

One way to handle inheritance is by having nested structs:

struct base
{
    ...
};

void method_of_base(base *b, ...);

struct child
{
    struct base base_elements;
    ...
};

You can then do calls like this:

struct child c;
method_of_base(&c.b, ...);

Answer 2

You can implement polymorphism with regular functions and virtual tables (vtables). Here's a pretty neat system that I invented (based on C++) for a programming exercise:

The constructors allocate memory and then call the class's init function where the memory is initialized. Each init function should also contain a static vtable struct that contains the virtual function pointers (NULL for pure virtual). Derived class init functions call the superclass init function before doing anything else.

A very nice API can be created by implementing the virtual function wrappers (not to be confused with the functions pointed to by the vtables) as follows (add static inline in front of it, if you do this in the header):

int playerGuess(Player* this) { return this->vtable->guess(this); }

Single inheritance can be done by abusing the binary layout of a struct:

Notice that multiple inheritance is messier as then you often need to adjust the pointer value when casting between types of the hierarchy.

Other type-specific data can be added to the virtual tables as well. Examples include runtime type info (e.g. type name as a string), linking to superclass vtable and the destructor chain. You probably want virtual destructors where derived class destructor demotes the object to its super class and then recursively calls the destructor of that and so on, until the base class destructor is reached and that finally frees the struct.

Encapsulation was done by defining the structs in player_protected.h and implementing the functions (pointed to by the vtable) in player_protected.c, and similarly for derived classes, but this is quite clumsy and it degrades performance (as virtual wrappers cannot be put to headers), so I would recommend against it.

Answer 3

Nice article & discussion regarding Objective-C here:

http://cocoawithlove.com/2009/10/objective-c-niche-why-it-survives-in.html

Answer 4

An interesting bit of history. Cfront, the original C++ implementation output C code and then requires a C compiler to actually build the final code. So, anything that could be expressed in C++ could be written as C.

Answer 5

Definitely look at Objective-C.

typedef struct objc_object {
    Class isa;
} *id;

typedef struct objc_class {
    struct objc_class *isa;
    struct objc_class *super_class
    const char *name;
    long version;
    long info
    long instance_size;
    struct objc_ivar_list *ivars;
    struct objc_method_list **methodLists;
   struct objc_cache *cache;
   struct objc_protocol_list *protocols;
} *Class;

As you can see, inheritance information, along with other details is held in a class struct (conveniently the class can also be treated as an object).

Objective-C suffers in the same manner as C++ with encapsulation in that you need to declare your variables publicly. Straight C is much more flexible in that you can just return void pointers that only your module has internal access to, so in that respect encapsulation is much better.

I once wrote a basic OO style C paint program as part of a graphics course - I didn't go as far as the class declaration, I simply used a vtable pointer as the first element of the struct and implemented hand-coded inheritance. The neat thing about playing around with vtables at such a low level is that you can change class behaviour at runtime by changing a few pointers, or change on objects class dynamically. It was quite easy to create all sorts of hybrid objects, fake multiple inheritance, etc.

Answer 6

the gtk and glib libraries use macros to cast objects to various types.
add_widget(GTK_WIDGET(myButton));
I can't say how it's done but you can read their source to find out exactly how it's done.