Okay, you may call me a noob but I'm realling confused.
My ex classmate paid me to write a program in C. She gave me the task and it said something like "blah blah blah make at least TWO CLASSES, write at least ONE CONSTRUCTOR and rewrite at least ONE METHOD" it says that word by word.
And then I told her "this is C++ not C" she said "but we're learning C"
I ignored it and wrote the program in c++ and sent to her as I thought she didn't know what she was talking about. She said "it doesn't work on code blocks, and wtf is cout <<" and then she sent me a chunk of code that they write and instead of cout and cin there was printf and scanf. It had to be C. So, I rewrote the program with printf and scanf and she still says codeblocks throw errors (I still left classes as task demanded).
I want to ask wtf? Does C have classes? Or is there a misunderstanding or something?
EDIT: I've come back to the question after so many years and noticed some a*****es took time to remove 99% text from the question. Get a life, this is not 1984 yet.
No, C doesn't have classes. That said, there are ways of simulating object-oriented programming in C - a quick Google search should yield some useful results.
No, C has no classes per se, only C++ (which started out as "C with classes" back then...). But you can use the standard C library in C++ code, even if it is often not considered good practice (where C++ has its own, higher level constructs, e.g. cout vs printf).
You can sort of emulate the behaviour of classes, inheritance and virtual functions in C too, but it's not worth the pain.
You should probably buy/get your ex classmate a C programming book :-)
C does not have the formal construct of a class. You can produce modules with module-level data that by your own agreement you will not extern anywhere else, or static data, and write functions to get, set, and otherwise manipulate that data. You can even go to the point of using function pointers to manipulate similar data types as if they were in a class.
However, you won't be protected by class semantics or other rules by the C compiler, because the C compiler does not know about classes. However, structuring your data is quite powerful.
C does not have classes.
But one can approximate a class by using static globals as private class members, and static functions as private member functions. extern members as public. In this case an entire file could be viewed as a class.
Probably this is not what you want.
A classic case of conflicting requirements, it seems :-)
The terminology of her requirements CLASS, CONSTRUCTOR, METHOD are all C++ terminology, while none of them is C terminology (the closest of which would arguably be STRUCT, INITIALIZATION, FUNCTION). Your friend is confusing something here. I doubt that her teacher is confusing something, though...
C does not have classes, but you can emulate it with structures and pointers to a function. C99 is a little bit (just a bit) based on C++, so it's easy to reproduce classes with C.
C doesn't support classes, but we do have tricky hacky workaround. Read entire explanation or simply scroll down to code section.
struct declaration:
I declared an struct with name class. Inside, I put uninitialized function with type void, name Print and one parameter char.
This will look like void class.Print(char);
struct don't allow initialization inside:
But struct doesn't allow initialization of variables inside. So we will init it outside. We created an a function with type of class, name Printer. It inits that struct and return initialized struct which we can easily make use of.
call the class:
Now we included helper header and declared variable with type class, name printer1, value Printer(). After I called member of class and print string using printer1.Print("Hello from class function");
main.c
#include "helper.h"
void main()
{
class printer1 = Printer();
printer1.Print("Hello from class function");
}
helper.h
#include <stdio.h> //imports int printf(...);
void print(const char* text)
{
printf(text);
}
typedef struct
{
void (*Print) (const char*);
} class;
class Printer()
{
class Printer;
Printer.Print = &print;
return Printer;
}
Note:
This exact example was compiled and tested successfully with VC and tcc compiler.
class is example name. You can use any other name as well.
C mostly uses functional/structural programming instead of implementing Object Oriented Programming as in languages like C++ , Java , Python etc. which use classes . But in few instances we use classes like in:
typedef struct {
ShapeClass shape;
float width, height;
}
RectangleClass;
Hope it helped.
Related
if I am developing a C shared library and I have my own structs. To make common operations on these struct instances easier for library consumers, can I provide function pointers to such functions inside the struct itself? Is it a good practice? Would there be issues with respect to multithreading where a utility function is called in parallel with different arguments and so on?
I know it goes a lot closer to C++ classes but I wish to stick to C and learn how it would be done in a procedural language as opposed to OOP.
To give an example
typedef struct tag tag;
typedef struct my_custom_struct my_custom_struct;
struct tag
{
// ...
};
struct my_custom_struct
{
tag *tags;
my_custom_struct* (*add_tag)(my_custom_struct* str, tag *tag);
};
my_custom_struct* add_tag(my_custom_struct* str, tag *tag)
{
// ...
}
where add_tag is a helper that manages to add the tag to tag list inside *str.
I saw this pattern in libjson-c like here- http://json-c.github.io/json-c/json-c-0.13.1/doc/html/structarray__list.html. There is a function pointer given inside array_list to help free it.
To make common operations on these struct instances easier for library
consumers, can I provide function pointers to such functions inside
the struct itself?
It is possible to endow your structures with members that are function pointers, pointing to function types whose parameters include pointers to your structure type, and that are intended to be used more or less like C++ instance methods, more or less as presented in the question.
Is it a good practice?
TL;DR: no.
The first problem you will run into is getting those pointer members initialized appropriately. Name correspondence notwithstanding, the function pointers in instances of your structure will not automatically be initialized to point to a particular function. Unless you make the structure type opaque, users can (and undoubtedly sometimes will) declare instances without calling whatever constructor-analog function you provide for the purpose, and then chaos will ensue.
If you do make the structure opaque (which after all isn't a bad idea), then you'll need non-member functions anyway, because your users won't be able to access the function pointers directly. Perhaps something like this:
struct my_custom_struct *my_add_tag(struct my_custom_struct *str, tag *tag) {
return str->add_tag(str, tag);
}
But if you're going to provide for that, then what's the point of the extra level of indirection? (Answer: the only good reason for that would be that in different instances, the function pointer can point to different functions.)
And similar applies if you don't make the structure opaque. Then you might suppose that users would (more) directly call
str->add_tag(str, tag);
but what exactly makes that a convenience with respect to simply
add_tag(str, tag);
?
So overall, no, I would not consider this approach a good practice in general. There are limited circumstances where it may make sense to do something along these lines, but not as a general library convention.
Would there be issues with
respect to multithreading where a utility function is called in
parallel with different arguments and so on?
Not more so than with functions designated any other way, except if the function pointers themselves are being modified.
I know it goes a lot closer to C++ classes but I wish to stick to C
and learn how it would be done in a procedural language as opposed to
OOP.
If you want to learn C idioms and conventions then by all means do so. What you are describing is not one. C code and libraries can absolutely be designed with use of OO principles such as encapsulation, and to some extent even polymorphism, but it is not conventionally achieved via the mechanism you describe. This answer touches on some of the approaches that are used for the purpose.
Is it a good practice?
TLDR; no.
Background:
I've been programming almost exclusively in embedded C on STM32 microcontrollers for the last year and a half (as opposed to using C++ or "C+", as I'll describe below). It's been very insightful for me to have to learn C at the architectural level, like I have. I've studied C architecture pretty hard to get to where I can say I "know C". It turns out, as we all know, C and C++ are NOT the same language. At the syntax level, C is almost exactly a subset of C++ (with some key differences where C supports stuff C++ does not), hence why people (myself included before this) frequently think/thought they are pretty much the same language, but at the architectural level they are VASTLY DIFFERENT ANIMALS.
Aside:
Note that my favorite approach to embedded is to use what some colloquially know as "C+". It is basically using a C++ compiler to write C-style embedded code. You basically just write C how you'd expect to write C, except you use C++ classes to vastly simplify the (otherwise pure C) architecture. In other words, "C+" is a pseudonym used to describe using a C++ compiler to write C-like code that uses classes instead of "object-based C" architecture (which is described below). You may also use some advanced C++ concepts on occasion, like operator overloading or templates, but avoid the STL for the most part to not accidentally use dynamic allocation (behind-the-scenes and automatically, like C++ vectors do, for example) after initialization, since dynamic memory allocation/deallocation in normal run-time can quickly use up scarce RAM resources and make otherwise-deterministic code non-deterministic. So-called "C+" may also include using a mix of C (compiled with the C compiler) and C++ (compiled with the C++ compiler), linked together as required (don't forget your extern "C" usage in C header files included in your C++ code, as required).
The core Arduino source code (again, the core, not necessarily their example "sketches" or example code for beginners) does this really well, and can be used as a model of good "C+" design. <== before you attack me on this, go study the Arduino source code for dozen of hours like I have [again, NOT the example "sketches", but their actual source code, linked-to below], and drop your "arduino is for beginners" pride right now.
The AVR core (mix of C and "C+"-style C++) is here: https://github.com/arduino/ArduinoCore-avr/tree/master/cores/arduino
Some of the core libraries ("C+"-style C++) are here: https://github.com/arduino/ArduinoCore-avr/tree/master/libraries
[aside over]
Architectural C notes:
So, regarding C architecture (ie: actual C, NOT "C+"/C-style C++):
C is not an OO language, as you know, but it can be written in an "object-based" style. Notice I say "object-based", NOT "object oriented", as that's how I've heard other pedantic C programmers refer to it. I can say I write object-based C architecture, and it's actually quite interesting.
To make object-based C architecture, here's a few things to remember:
Namespaces can be done in C simply by prepending your namespace name and an underscore in front of something. That's all a namespace really is after-all. Ex: mylibraryname_foo(), mylibraryname_bar(), etc. Apply this to enums, for example, since C doesn't have "enum classes" like C++. Apply it to all C class "methods" too since C doesn't have classes. Apply to all global variables or defines as well that pertain to a particular library.
When making C "classes", you have 2 major architectural options, both of which are very valid and widely used:
Use public structs (possibly hidden in headers named "myheader_private.h" to give them a pseudo-sense of privacy)
Use opaque structs (frequently called "opaque pointers" since they are pointers to opaque structs)
When making C "classes", you have the option of wrapping up pointers to functions inside of your structs above to give it a more "C++" type feel. This is somewhat common, but in my opinion a horrible idea which makes the code nearly impossible to follow and very difficult to read, understand, and maintain.
1st option, public structs:
Make a header file with a struct definition which contains all your "class data". I recommend you do NOT include pointers to functions (will discuss later). This essentially gives you the equivalent of a "C++ class where all members are public." The downside is you don't get data hiding. The upside is you can use static memory allocation of all of your C "class objects" since your user code which includes these library headers knows the full specification and size of the struct.
2nd option: opaque structs:
In your library header file, make a forward declaration to a struct:
/// Opaque pointer (handle) to C-style "object" of "class" type mylibrarymodule:
typedef struct mylibrarymodule_s *mylibrarymodule_h;
In your library .c source file, provide the full definition of the struct mylibrarymodule_s. Since users of this library include only the header file, they do NOT get to see the full implementation or size of this opaque struct. That is what "opaque" means: "hidden". It is obfuscated, or hidden away. This essentially gives you the equivalent of a "C++ class where all members are private." The upside is you get true data hiding. The downside is you can NOT use static memory allocation for any of your C "class objects" in your user code using this library, since any user code including this library doesn't even know how big the struct is, so it cannot be statically allocated. Instead, the library must do dynamic memory allocation at program initialization, one time, which is safe even for embedded deterministic real-time safety-critical systems since you are not allocating or freeing memory during normal program execution.
For a detailed and full example of Option 2 (don't be confused: I call it "Option 1.5" in my answer linked-to here) see my other answer on opaque structs/pointers here: Opaque C structs: how should they be declared?.
Personally, I think the Option 1, with static memory allocation and "all public members", may be my preferred approach, but I am most familiar with the opaque struct Option 2 approach, since that's what the C code base I work in the most uses.
Bullet 3 above: including pointers to functions in your structs.
This can be done, and some do it, but I really hate it. Don't do it. It just makes your code so stinking hard to follow. In Eclipse, for instance, which has an excellent indexer, I can Ctrl + click on anything and it will jump to its definition. What if I want to see the implementation of a function I'm calling on a C "object"? I Ctrl + click it and it jumps to the declaration of the pointer to the function. But where's the function??? I don't know! It might take me 10 minutes of grepping and using find or search tools, digging all around the code base, to find the stinking function definition. Once I find it, I forget where I was, and I have to repeat it all over again for every single function, every single time I edit a library module using this approach. It's just bad. The opaque pointer approach above works fantastic instead, and the public pointer approach would be easy too.
Now, to directly answer your questions:
To make common operations on these struct instances easier for library consumers, can I provide function pointers to such functions inside the struct itself?
Yes you can, but it only makes calling something easier. Don't do it. Finding the function to look at its implementation becomes really hard.
Is it a good practice?
No, use Option 1 or Option 2 above instead, where you now just have to call C "namespaced" "methods" on every C "object". You must simply pass the "members of the C class" into the function as the first argument for every call instead. This means instead of in C++ where you can do:
myclass.dosomething(int a, int b);
You'll just have to do in object-based C:
// Notice that you must pass the "guts", or member data
// (`mylibrarymodule` here), of each C "class" into the namespaced
// "methods" to operate on said C "class object"!
// - Essentially you're passing around the guts (member variables)
// of the C "class" (which guts are frequently referred to as
// "private data", or just `priv` in C lingo) to each function that
// needs to operate on a C object
mylibrarymodule_dosomething(mylibrarymodule_h mylibrarymodule, int a, int b);
Would there be issues with respect to multithreading where a utility function is called in parallel with different arguments and so on?
Yes, same as in any multithreaded situation where multiple threads are trying to access the same data. Just add a mutex to each C struct-based "object", and be sure each "method" acting on your C "objects" properly locks (takes) and unlocks (gives) the mutex as required before operating on any shared volatile members of the C "object".
Related:
Opaque C structs: how should they be declared? [use "Object-based" C architecture]
I would like to suggest you reading com specification, you will gain a lot. all these com, ole and dcom technology is based on a simple struct that incorporates its own data and methods.
https://www.scribd.com/document/45643943/Com-Spec
simplied more here
http://www.voidcn.com/article/p-fixbymia-beu.html
I'm learning C at the moment and have just covered structs. It seems like structs can only have members but no methods restricted to a given struct. Suppose I want to write an add method for a linked list.
In Java, I'd make a linked list class and write a method within that class:
public void add(Node toAdd) {...}
In C, I can write a function that takes in a linked list or a pointer to a linked list and modifies it by adding a node. But is there any way to do this more similarly to how Java does it?
I'm worried I'm going to have a massive file of functions with little organization otherwise...or is this just a pain of C and something that Obj-C improves upon?
Yes there is a way to do it in C. The specific answer that you are looking for is called function pointer.
You may declare a function pointer in the struct which links to a function that you have defined somewhere else in the file.
The following stackoverflow discussion is relevant to this.
here
Here is another relevant discussion that points out that C struct does not support functions like what X3lif has suggested. And in the discussion, the solution is function pointer.
C - function inside struct
You are probably confusing some of the concepts here. You can not compare Java with C, but you can compare Java with C++.
C is meant for low level programming and does not support object oriented programming.
C++ is an extension of C with object oriented features.
So what you are searching for does not exist in C, but in C++. In C++ you can declare classes:
class Foo {
int member;
void addToMember(int x) {
member += x;
}
};
In C, there are no classes. Usually you define a struct and create a set of functions operating on this struct:
struct Foo {
int member;
}
void fooInit(Foo *foo);
void fooAddToMember(Foo *foo, int x);
This is actually an old way, how some kind of object oriented programming was done with languages which didn't supported objects.
Today, nobody would do this anymore. One would simple use C++ for object oriented programming. There is actually no need to stick with C. Even most micro controllers today support C++ or other object oriented languages and even modern operating system kernels are moving away from pure C.
I read plenty of questions regarding
declaration of functions inside structure?
But I did not get a satisfactory answer.
My question is not about whether functions can be declared inside a structure or not?
Instead, my question is
WHY function can not be declared inside structure?
Well that is the fundamental difference between C and C++ (works in C++). C++ supports classes (and in C++ a struct is a special case of a class), and C does not.
In C you would implement the class as a structure with functions that take a this pointer explicitly, which is essentially what C++ does under the hood. Coupled with a sensible naming convention so you know what functions belong to which classes (again something C++ does under then hood with name-mangling), you get close to object-based if not object-oriented programming. For example:
typedef struct temp
{
int a;
} classTemp ;
void classTemp_GetData( classTemp* pThis )
{
printf( "Enter value of a : " );
scanf( "%d", &(pThis->a) );
}
classTemp T ;
int main()
{
classTemp_GetData( &T );
}
However, as you can see without language support for classes, implementing then can become tiresome.
In C, the functions and data structures are more or less bare; the language gives a minimum of support for combining data structures together, and none at all (directly) for including functions with those data structures.
The purpose of C is to have a language that translates as directly as possible into machine code, more like a portable assembly language than a higher-level language such as C++ (not that C++ is all that high-level). C let's you get very close to the machine, getting into details that most languages abstract away; the down side of this is that you have to get close to the machine in C to use the language to its utmost. It takes a completely different approach to programming from C++, something that the surface similarities between them hide.
Check out here for more info (wonderful discussions there).
P.S.: You can also accomplish the functionality by using function pointers, i.e.
have a pointer to a function (as a variable) inside the struct.
For example:
#include <stdio.h>
struct t {
int a;
void (*fun) (int * a); // <-- function pointers
} ;
void get_a (int * a) {
printf (" input : ");
scanf ("%d", a);
}
int main () {
struct t test;
test.a = 0;
printf ("a (before): %d\n", test.a);
test.fun = get_a;
test.fun(&test.a);
printf ("a (after ): %d\n", test.a);
return 0;
}
WHY function can not be declared inside structure?
Because C standard does not allow to declare function/method inside a structure. C is not object-oriented language.
6.7.2.1 Structure and union specifiers:
A structure or union shall not contain a member with incomplete or function type(hence,
a structure shall not contain an instance of itself, but may contain a pointer to an instance
of itself), except that the last member of a structure with more than one named member
may have incomplete array type; such a structure (and any union containing, possibly
recursively, a member that is such a structure) shall not be a member of a structure or an
element of an array.
I suppose there were and are many reasons, here are several of them:
C programming language was created in 1972, and was influenced by pure assembly language, so struct was supposed as "data-only" element
As soon as C is NOT object oriented language - there is actually no sense to define functions inside "data structure", there are no such entity as constructor/method etc
Functions are directly translated to pure assembly push and call instructions and there are no hidden arguments like this
I guess, because it wouldn't make much sence in C. If you declare a function inside structure, you expect it to be somehow related to that structure, right? Say,
struct A {
int foo;
void hello() {
// smth
}
}
Would you expect hello() to have access to foo at least? Because otherwise hello() only got something like namespace, so to call it we would write A.hello() - it would be a static function, in terms of C++ - not much difference from normal C function.
If hello() has access to foo, there must be a this pointer to implement such access, which in C++ always implicitly passed to functions as first argument.
If a structure function has access to structure variables, it must be different somehow from access that have other functions, again, to add some sence to functions inside structures at all. So we have public, private modificators.
Next. You don't have inheritance in C (but you can simulate it), and this is something that adds lots of sence to declaring functions inside structutes. So here we'd like to add virtual and protected.
Now you can add namespaces and classes, and here you are, invented C++ (well, without templates).
But C++ is object-oriented, and C is not. First, people created C, then they wrote tons of programs, understood some improvements that could be made, and then, following reasonings that I mentioned earlier, they came up with C++. They did not change C instead to separate concepts - C is procedure-oriented, and C++ is object-oriented.
C was designed so that it could be processed with a relatively simple compilation system. To allow function definitions to appear within anything else would have required the compiler to keep track of the context in which the function appeared while processing it. Given that members of a structure, union, or enum declaration do not enter scope until the end of the declaration, there would be nothing that a function declared within a structure could do which a function declared elsewhere could not.
Conceptually, it might have been nice to allow constant declarations within a structure; a constant pointer to a literal function would then be a special case of that (even if the function itself had to be declared elsewhere), but that would have required the C compiler to keep track of more information for each structure member--not just its type and its offset, but also whether it was a constant and, if so, what its value should be. Such a thing would not be difficult in today's compilation environments, but early C compilers were expected to run with orders of magnitude less memory than would be available today.
Given that C has been extended to offer many features which could not very well be handled by a compiler running on a 64K (or smaller) system, it might reasonably be argued that it should no longer be bound by such constraints. Indeed, there are some aspect of C's heritage which I would like to lose, such as the integer-promotion rules which require even new integer types to follow the inconsistent rules for old types, rather than allowing the new types to have explicitly specified behavior [e.g. have a wrap32_t which could be converted to any other integer type without a typecast, but when added to any "non-wrap" integer type of any size would yield a wrap32_t]. Being able to define functions within a struct might be nice, but it would be pretty far down on my list of improvements.
Somebody can give me an example to do this. Suppose static variable scope is limited to file only.That is private to that file.Like that some more examples i want to know. In other words HOW TO ACHIEVE DATA HIDING CONCEPT IN C-LANGUAGE WITH CURRENTLY AVAILABLE KEYWORDS(STRUCT,STATIC...ETC)
This guy is one of the worlds authorities on embedded systems. He wrote this white paper on OOP in c.
http://www.state-machine.com/resources/cplus_3.0_manual.pdf
You can use a private header (say xyz_private.h) where you define your private structs (say struct xyz_private_t). In the public header you canthen do
typedef struct xyz_private_t *xyz_ptr_t; or
typedef struct {
... some public members ...
struct xyz_private_t *private;
} mytype_t;
This is similiar to PIMPL in C++.
private, friend, protected can not be distinguished - either a file can/does access the private header or it can't/doesn't.
You would have to rewrite/extend the compiler, add new grammar to the lexigraphic unit (this is probably the easiest part) and most importantly, add new specs. If you would add these keywords to the C-Language, you wouldn't have the C-Language anymore, but a derivate. Your code wouldn't be understood by any other compiler. Also, those aren't just keywords, they are specific expected behaviour, you can't just implement the keywords, you'd have to add full OOP support to the language. If this is what you are planning - good luck.
Data-hiding is a feature of managed languages, because there is a runtime seperate to your program taking care of things behind the curtain, this does not exist in C. If you want to have these features (data-hiding and other abstractions limited to managed languages), you'd have to exactly produce such a construct, ie. implementing a runtime to take care of things behind the curtain. Then, and ONLY then, are you able to achieve data hiding and other abstractions.
C simply does not offer these possibilites, any code in the program can access any address in the virtual memory of the process, going around anything you are planing.
My advice, even if it will be hard for you: use a managed language for that. C isn't the right language to achieve what you want to do.
Yes, internal linkage (static) effectively makes things private to a file. You don't necessarily need that, though. You can simulate a class with private members by defining a struct inside the source file and providing only a typedef and a set of functions to your users. All of the functions, except for the "constructor" would take an explicit "this" argument. Your faux constructor would simply allocate an instance of your type and return the pointer. Since all your functions would be defined (with external linkage) in the same source as the struct, they can see the members. Your users who only see the typedef and function prototypes cannot.
You can use static in this case to emulate a singleton by having your functions return a pointer to your internally declared instance.
C++ originated as a pre-compiler for C with pre-compilers such as Glockenspiel and CFront. You may find what you are looking for here (it also provide sources).
If you don't pick up clues from there, I think the only way you can achieve true data-hiding in C is to declare your variables as static variables to functions.
This question already has answers here:
Object Oriented pattern in C? [duplicate]
(4 answers)
Closed 1 year ago.
Possible Duplicates:
Can you write object oriented code in C?
Object Oriented pattern in C ?
I remember reading a while ago about someone (I think it was Linus Torvalds) talking about how C++ is a horrible language and how you can write object-oriented programs with C. On having had time to reflect, I don't really see how all object oriented concepts carry over into C. Some things are fairly obvious. For example:
To emulate member functions, you can put function pointers in structs.
To emulate polymorphism, you can write a function that takes a variable number of arguments and do some voodoo depending on, say, the sizeof the parameter(s)
How would you emulate encapsulation and inheritance though?
I suppose encapsulation could sort of be emulated by having a nested struct that stored private members. It would be fairly easy to get around, but could perhaps be named PRIVATE or something equally obvious to signal that it isn't meant to be used from outside the struct. What about inheritance though?
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.
Have you read the "bible" on the subject? See Object Oriented C...
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.
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.
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.
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.
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()).
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.
Nice article & discussion regarding Objective-C here:
http://cocoawithlove.com/2009/10/objective-c-niche-why-it-survives-in.html
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/
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.
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, ...);
You might want to look at Objective-C, that's pretty much what it does. It's just a front-end that compiles Objective-C OO code to C.