C - function inside struct

Question

Im trying to create a function inside a structure  so far i have this code   typedef struct client t client t   pno  struct client t           pid t pid          char password TAM MAX      - gt  50 chars         pno next           pno AddClient                              code                   int main          client t client         code         client AddClient         Error  client h 24 2  error  expected                                    or    attribute    before         token   Which is the correct way to do it

User · Answer

You are trying to group code according to struct. C grouping is by file. You put all the functions and internal variables in a header or a header and a object ".o" file compiled from a c source file.

It is not necessary to reinvent object-orientation from scratch for a C program, which is not an object oriented language.

I have seen this before. It is a strange thing. Coders, some of them, have an aversion to passing an object they want to change into a function to change it, even though that is the standard way to do so.

I blame C++, because it hid the fact that the class object is always the first parameter in a member function, but it is hidden. So it looks like it is not passing the object into the function, even though it is.

Client.addClient(Client& c); // addClient first parameter is actually 
                             // "this", a pointer to the Client object.

C is flexible and can take passing things by reference.

A C function often returns only a status byte or int and that is often ignored. In your case a proper form might be

 err = addClient( container_t  cnt, client_t c);
 if ( err != 0 )
   { fprintf(stderr, "could not add client (%d) \n", err );

addClient would be in Client.h or Client.c

User · Answer

As others have noted  embedding function pointers directly inside your structure is usually reserved for special purposes  like a callback function   What you probably want is something more like a virtual method table   typedef struct client ops t client ops t  typedef struct client t client t   pno   struct client t                     client ops t  ops      struct client ops t       pno   AddClient  client t         pno   RemoveClient  client t         pno AddClient  client t  client    return client- gt ops- gt AddClient client     pno RemoveClient  client t  client    return client- gt ops- gt RemoveClient client       Now  adding more operations does not change the size of the client t structure  Now  this kind of flexibility is only useful if you need to define many kinds of clients  or want to allow users of your client t interface to be able to augment how the operations behave   This kind of structure does appear in real code  The OpenSSL BIO layer looks similar to this  and also UNIX device driver interfaces have a layer like this

User · Answer

This will only work in C     Functions in structs are not a feature of C   Same goes for your client AddClient    call     this is a call for a member function  which is object oriented programming  i e  C     Convert your source to a  cpp file and make sure you are compiling accordingly   If you need to stick to C  the code below is  sort of  the equivalent   typedef struct client t client t   pno  struct client t           pid t pid          char password TAM MAX      - gt  50 chars         pno next        pno AddClient pno  pclient            code        int main          client t client         code         AddClient client

User · Answer

You can pass the struct pointer to function as function argument  It called pass by reference  If you modify something inside that pointer  the others will be updated to  Try like this  typedef struct client t client t   pno  struct client t           pid t pid          char password TAM MAX      - gt  50 chars         pno next      pno AddClient client t  client               this will change the original client value            client password    quot secret quot      int main         client t client         code         AddClient  amp client

User · Answer

It can t be done directly  but you can emulate the same thing using function pointers and explicitly passing the  this  parameter   typedef struct client t client t   pno  struct client t           pid t pid          char password TAM MAX      - gt  50 chars         pno next           pno   AddClient  client t             pno client t AddClient client t  self       code       int main          client t client      client AddClient   client t AddClient     probably really done in some init fn        code         client AddClient  amp client        It turns out that doing this  however  doesn t really buy you an awful lot  As such  you won t see many C APIs implemented in this style  since you may as well just call your external function and pass the instance

User · Answer

How about this    include  lt stdio h gt   typedef struct hello       int   someFunction       hello   int foo         return 0     hello Hello         struct hello aHello      aHello someFunction    amp foo      return aHello     int main         struct hello aHello   Hello        printf  Print hello   d n   aHello someFunction          return 0

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