Cocoa Enumeration

Jonathan Rentzsch is back with a post about the macros he uses to tame Cocoa’s enumeration idiom. Here’s the macro I’ve been using:

#define foreach(object, enumerator) 
    id mjtForeachEnumerator ## object = (enumerator); 
    if ( [mjtForeachEnumerator ## object respondsToSelector:@selector(objectEnumerator)] ) 
        mjtForeachEnumerator ## object = [mjtForeachEnumerator ## object objectEnumerator]; 
    SEL mjtNextObjectSEL ## object = @selector(nextObject); 
    IMP mjtNextObjectIMP ## object = [mjtForeachEnumerator ## object methodForSelector:mjtNextObjectSEL ## object]; 
    id object; 
    while ( (mjtForeachEnumerator ## object) && 
            object = mjtNextObjectIMP ## object(mjtForeachEnumerator ## object, mjtNextObjectSEL ## object) )

• Like Rentzsch’s macro, this will call -objectEnumerator for you. Rentzsch looks for objects that are NSEnumerators and doesn’t call -objectEnumerator in that case; I look for objects that support -objectEnumerator and do call it in that case.
• Unlike Rentzsch’s macro, the control variables are not local to the loop. His way is better, but at the time I wrote this, it either wasn’t possible to get the Objective-C compiler in C99 mode (without using Objective-C++) or I didn’t know how to do it. Thus, I glue the element name to the end of each control variable name, to try to make it unique. This lets me use foreach more than once in a single scope.
• After using the macro for six months or so, I added the code to cache the IMP for -nextObject. All the code using foreach took advantage of the change with just a recompile. The speed increase was noticeable (without a stopwatch) in some cases. (In retrospect, I should probably have just used @selector(nextObject) instead of mjtNextObjectSEL, since @selector happens at link time, not runtime.)
• We use the opposite parameter order. My macro is used like: foreach( word, words ), which means foreach word in words. (In fact, if you really wanted to, you could #define in to be a comma and then write foreach ( word in words ), but I don’t recommend that.)

Cocoa Macros Objective-C Optimization Programming