Re: Embed ObjC in an API library?

[Pascal Bourguignon]

Helge Hess writes:
> Kaelin Colclasure wrote:
> > Is it possible to implement a set of functions in Objective C, and then 
> > wrap them with a plain-old C interface and package them as a shared 
> > library?
> 
> Yes. You actually only need to wrap them to make it more convenient for 
> the C-developer. You can call any Objective-C method/class in ANSI-C by 
> using the Objective-C runtime, eg:
> 
>    #include <objc/objc.h>
> 
>    objc_msg_lookup(myObject, sel_get_uid("doIt:"))
>      (myObject, myArg);

Well, Kaelin asked  for functions, not for objects  and methods.  When
you declare a function in Objective-C, you get exactly the function in
C, because  Objective-C is  a strict superset  of C.  There's  no name
mangling like  in C++. So you  can write C function  in an Objective-C
source (even inside @implementation/@end,  this has nothing to do with
any  name  scoping),  and call  them  from  C  (or  even from  C++  if
encapsulating the  #include of the  header inside extern  "C"{...} (or
extern "Objective-C"{...} if the  compiler accepts it)). 

--------------<c-interface-to-module.h>------------
// Objective-C header
#ifndef c_interface_to_module_h
#define c_interface_to_module_h
    extern void* module_func0(void)
    extern int module_func1(void* ref,int arg);
    extern int module_func2(void* ref);
    extern int module_func3(int arg);
#endif
--------------<module.m>---------------------------
// Objective-C source
#include <SomeClass.h>
    void* objc_module_func0(void)
    {     
        return([[SomeClass alloc] init]);
    }
    int objc_module_func1(void* ref,int arg)
    {
        return([[(SomeClass*)ref method1:arg]]);
    }
    int objc_module_func2(void* ref);
    {
        return([[(SomeClass*)ref method2]]);
    }
    int objc_module_func3(int arg);
    {
        return(expr(arg));
    }
--------------<client.c>---------------------------
#include <c-inteface-to-module.h>

    void fun(void){
    {    
         void* ref=moudle_func0();
         module_func1(ref,module_func3());
         module_func2(ref);
    }
-------------------------------------------


Note that if what you want to encapsulate is message passing, doing it
in an  Objective-C source  allow you to  use the  [object message:arg]
notation instead of  calling the low level API. That  API should be of
use only  for dynamic gluing. For  example if you want  to write stuff
like that:

     char* action=compute_or_input_some_method_selector();
     id    sender=find_some_object();
     id    target=compute_some_object(sender);
     imp   method=objc_msg_lookup(target,sel_get_uid(action));
     method(target,sender);



-- 
__Pascal_Bourguignon__                   http://www.informatimago.com/
----------------------------------------------------------------------
Do not adjust your mind, there is a fault in reality.