How do you implement a class in C

Question

Assuming I have to use C  no C   or object oriented compilers  and I don t have dynamic memory allocation  what are some techniques I can use to implement a class  or a good approximation of a class  Is it always a good idea to isolate the  class  to a separate file  Assume that we can preallocate the memory by assuming a fixed number of instances  or even defining the reference to each object as a constant before compile time  Feel free to make assumptions about which OOP concept I will need to implement  it will vary  and suggest the best method for each   Restrictions    I have to use C and not an OOP because I m writing code for an embedded system  and the compiler and preexisting code base is in C   There is no dynamic memory allocation because we don t have enough memory to reasonably assume we won t run out if we start dynamically allocating it  The compilers we work with have no problems with function pointers

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My strategy is    Define all code for the class in a separate file Define all interfaces for the class in a separate header file All member functions take a  ClassHandle  which stands in for the instance name  instead of o foo    call foo oHandle  The constructor is replaced with a function void ClassInit ClassHandle h  int x  int y      OR ClassHandle ClassInit int x  int y      depending on the memory allocation strategy All member variables are store as a member of a static struct in the class file  encapsulating it in the file  preventing outside files from accessing it The objects are stored in an array of the static struct above  with predefined handles  visible in the interface  or a fixed limit of objects that can be instantiated If useful  the class can contain public functions that will loop through the array and call the functions of all the instantiated objects  RunAll   calls each Run oHandle  A Deinit ClassHandle h  function frees the allocated memory  array index  in the dynamic allocation strategy   Does anyone see any problems  holes  potential pitfalls or hidden benefits drawbacks to either variation of this approach  If I am reinventing a design method  and I assume I must be   can you point me to the name of it

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That depends on the exact  object-oriented  feature-set you want to have  If you need stuff like overloading and or virtual methods  you probably need to include function pointers in structures   typedef struct     float   computeArea  const ShapeClass  shape     ShapeClass   float shape computeArea const ShapeClass  shape      return shape- gt computeArea shape       This would let you implement a class  by  inheriting  the base class  and implementing a suitable function   typedef struct     ShapeClass shape    float width  height    RectangleClass   static float rectangle computeArea const ShapeClass  shape      const RectangleClass  rect    const RectangleClass    shape    return rect- gt width   rect- gt height      This of course requires you to also implement a constructor  that makes sure the function pointer is properly set up  Normally you d dynamically allocate memory for the instance  but you can let the caller do that  too   void rectangle new RectangleClass  rect      rect- gt width   rect- gt height   0 f    rect- gt shape computeArea   rectangle computeArea      If you want several different constructors  you will have to  decorate  the function names  you can t have more than one rectangle new   function   void rectangle new with lengths RectangleClass  rect  float width  float height      rectangle new rect     rect- gt width   width    rect- gt height   height      Here s a basic example showing usage   int main void      RectangleClass r1     rectangle new with lengths  amp r1  4 f  5 f     printf  rectangle r1 s area is  f units square n   shape computeArea  amp r1      return 0      I hope this gives you some ideas  at least  For a successful and rich object-oriented framework in C  look into glib s GObject library   Also note that there s no explicit  class  being modelled above  each object has its own method pointers which is a bit more flexible than you d typically find in C    Also  it costs memory  You could get away from that by stuffing the method pointers in a class structure  and invent a way for each object instance to reference a class

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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

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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- gt uuid   uuid new         return 0    uuid t Object get uuid Object  self       Don t actually create getters in C        return self- gt uuid    int Object clean Object  self        uuid free self- gt uuid        return 0         Person c  include  person h   int Person init Person  self  char  name        Object init  amp self- gt obj      Or just Object init  amp self       self- gt name   strdup name        return 0    int Person greet Person  self        printf  Hello   s   self- gt name        return 0    int Person clean Person  self        free self- gt name       Object clean self        return 0         main c int main void        Person p       Person init  amp p   John        Person greet  amp p       Object get uuid  amp p      Inherited function     Person clean  amp 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  amp p

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The first c   compiler actually was a preprocessor which translated the C   code into C   So it s very possible to have classes in C  You might try and dig up an old C   preprocessor and see what kind of solutions it creates

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If you only want one class  use an array of structs as the  objects  data and pass pointers to them to the  member  functions  You can use typedef struct  whatever Whatever before declaring struct  whatever to hide the implementation from client code  There s no difference between such an  object  and the C standard library FILE object   If you want more than one class with inheritance and virtual functions  then it s common to have pointers to the functions as members of the struct  or a shared pointer to a table of virtual functions  The GObject library uses both this and the typedef trick  and is widely used    There s also a book on techniques for this available online - Object Oriented Programming with ANSI C

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C Interfaces and Implementations  Techniques for Creating Reusable Software  David R  Hanson     http   www informit com store product aspx isbn 0201498413   This book does an excellent job of covering your question   It s in the Addison Wesley Professional Computing series   The basic paradigm is something like this      for data structure foo     FOO  myfoo  myfoo   foo create       foo something myfoo        myfoo   foo append myfoo        foo delete myfoo

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you can take a look at GOBject  it s an OS library that give you a verbose way to do an object   http   library gnome org devel gobject stable

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I had to do it once too for a homework  I followed this approach    Define your data members in a struct  Define your function members that take a pointer to your struct as first argument  Do these in one header  amp  one c  Header for struct definition  amp  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

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In your case the good approximation of the class could be the an ADT  But still it won t be the same

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Use a struct to simulate the data members of a class  In terms of method scope you can simulate private methods by placing the private function prototypes in the  c file and the public functions in the  h file

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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

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My approach would be to move the struct and all primarily-associated functions to a separate source file s  so that it can be used  portably    Depending on your compiler  you might be able to include functions into the struct  but that s a very compiler-specific extension  and has nothing to do with the last version of the standard I routinely used

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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

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include  lt stdio h gt   include  lt math h gt   include  lt string h gt   include  lt uchar h gt          Define Shape class     typedef struct Shape Shape  struct Shape                  Variables header                double width  height                  Functions header                double   area  Shape  shape              Functions     double calc Shape  shape            return shape- gt width   shape- gt height            Constructor     Shape  Shape         Shape s       s width   1      s height   1       s area   calc       return s                                                     int main         Shape s1    Shape        s1 width   5 35      s1 height   12 5462       printf  Hello World n n         printf  User width    f n   s1 width       printf  User height    f n   s1 height       printf  User area    f n n   s1 area  amp s1         printf  Made with  xe2 x99 xa5  n         return 0

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Do you want virtual methods     If not then you just define a set of function pointers in the struct itself   If you assign all the function pointers to standard C functions then you will be able to call functions from C in very similar syntax to how you would under C     If you want to have virtual methods it gets more complicated   Basically you will need to implement your own VTable to each struct and assign function pointers to the VTable depending on which function is called   You would then need a set of function pointers in the struct itself that in turn call the function pointer in the VTable   This is  essentially  what C   does   TBH though     if you want the latter then you are probably better off just finding a C   compiler you can use and re-compiling the project   I have never understood the obsession with C   not being usable in embedded   I ve used it many a time and it works is fast and doesn t have memory problems   Sure you have to be a bit more careful about what you do but its really not that complicated

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

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