Passing by value

Question

Hi all,
Can anyone please tell me how can we achieve passing by value in objective-c?
I am using:
NSMutableString *fileNames;
//do some operation on fileNames
\[self printData:fileNames\];
-(void)printData: (NSString*)dataToPrint
//some code
Is this passing by reference or passing by value??
If it is passing by reference then how can we achieve passing by value??
Thanks and Regards,
$hr!k@nt

Answer 1

In Objective-C, objects are ALWAYS treated as "reference to an object", so passing an objects by value like in C++ is "impossible" in Objective-C, like in Java and C# (which have adopted that system from Objective-C).
The compiler won't accept that you pass the object by value, that means you can't take the star "*" off to remove the reference.
The bycopy keyword is a different matter, it's here for remote messaging optimization. When you do not need a passed object to be modified by the callee you pass it "bycopy" to tell the compiler that the new object value needs not to be sent back to the caller.
However, if you really need an object passed by value there's a hack using C-structures.
If you're using Objective-C 1.0 (pre-Leopard) you can use the @defs directive like that :
typedef struct MyClassStruct {
@defs(MyClass);
} MyClassStruct;
MyClass *obj = [[MyClass alloc] init];
MyClassStruct myObj = *(MyClassStruct *)obj;
That's not valid in Objective-C 2.0 (Leopard and later) because @defs is deprecated.
However, you can do manually what @defs do automatically but that's not portable. You just need to define a struct with all the instance variable of the class and its super classes in the same order, for example :
@interface MyClass : NSObject {
int value;
char *string;
@end
typedef struct MyClassStruct {
Class isa;
int value;
char *string;
} MyClassStruct;
After all of that, you just need to put your struct type as the argument type. And you can then pass the objects "by value"... But that's an horrible, completely anti-OOP, non-portable hack which shouldn't be used.