Object oriented programming in C [duplicate]

Answer 1

Take a look at the way the VFS layer works in the Linux kernel for an example of an inheritance pattern. The file operations for the various filesystems "inherit" a set of generic file operations functions (eg generic_file_aio_read(), generic_file_llseek()...), but can override them with their own implementations (eg. ntfs_file_aio_write()).

Answer 2

From a little bit higher altitude and considering the problem rather more open-minded than as the OOP mainstream may suggest, Object-Oriented Programming means thinking about objects as of data with associated functions. It does not necessarily mean an function has to be physically attached to an object as it is in popular languages which support paradigm of OOP, for instance in C++:

struct T
{
   int data;
   int get_data() const { return data; }
};

I would suggest to take a closer look at GTK+ Object and Type System. It is a brilliant example of OOP realised in C programming language:

GTK+ implements its own custom object system, which offers standard object-oriented features such as inheritance and virtual function

The association can also be contractual and conventional.

Regarding encapsulation and data hiding techniques, popular and simple one may be Opaque Pointer (or Opaque Data Type) - you can pass it around but in order to load or store any information, you have to call associated function which knows how to talk to the object hidden behind that opaque pointer.

Another one, similar but different is Shadow Data type - check this link where Jon Jagger gives excellent explanation of this not-so-well-known-technique.

Answer 3

Have you read the "bible" on the subject? See Object Oriented C...

Answer 4

For a great example of object-oriented programming in C, look at the source of POV-Ray from several years ago - version 3.1g is particularly good. "Objects" were struct with function pointers, of course. Macros were used to provide the core methods and data for an abstract object, and derived classes were structs that began with that macro. There was no attempt to deal with private/public, however. Things to be seen were in .h files and implementation details were in .c files, mostly, except for a lot of exceptions.

There were some neat tricks that I don't see how could be carried over to C++ - such as converting one class to a different but similar one on the fly just by reassigning function pointers. Simple for today's dynamic languages. I forgot the details; I think it might have been CSG intersection and union objects.

http://www.povray.org/

Answer 5

How would you emulate encapsulation and inheritance though?

Actually, encapsulation is the easiest part. Encapsulation is a design philosophy, it has nothing at all to do with the language and everything to to with how you think about problems.

For example, the Windows FILE api is completely encapsulated. When you open a file, you get back an opaque object that contains all of the state information for the file 'object'. You hand this handle back to each of the file io apis. The encapsulation is actually much better than C++ because there is no public header file that people can look at and see the names of your private variables.

Inheritance is harder, but it isn't at all necessary in order for your code to be object oriented. In some ways aggregation is better than inheritance anyway, and aggregation is just as easy in C as in C++. see this for instance.

In response to Neil see Wikipedia for an explanation of why inheritance isn't necessary for polymorphism.

Us old-timers wrote object oriented code years before C++ compilers were available, it's a mind-set not a tool-set.

Answer 6

Apple's C-based CoreFoundation framework was actually written so that its "objects" could double as objects in Objective-C, an actual OO language. A fairly large subset of the framework is open source on Apple's site as CF-Lite. Might be a useful case study in a major OS-level framework done this way.