NSString stringWithFormat swizzled to allow missing format numbered args

Question

Based on this SO question asked a few hours ago, I have decided to implement a swizzled method that will allow me to take a formatted NSString as the format arg into stringWithFormat, and have it not break when omitting one of the numbered arg references (%1$@, %2$@)

I have it working, but this is the first copy, and seeing as this method is going to be potentially called hundreds of thousands of times per app run, I need to bounce this off of some experts to see if this method has any red flags, major performance hits, or optimizations

#define NUMARGS(...)  (sizeof((int[]){__VA_ARGS__})/sizeof(int))
@implementation NSString (UAFormatOmissions)
+ (id)uaStringWithFormat:(NSString *)format, ... {  
    if (format != nil) {
        va_list args;
        va_start(args, format);

        // $@ is an ordered variable (%1$@, %2$@...)
        if ([format rangeOfString:@"$@"].location == NSNotFound) {
            //call apples method
            NSString *s = [[[NSString alloc] initWithFormat:format arguments:args] autorelease];
            va_end(args);
            return s;
        }

        NSMutableArray *newArgs = [NSMutableArray arrayWithCapacity:NUMARGS(args)];
        id arg = nil;
        int i = 1;
        while (arg = va_arg(args, id)) {
            NSString *f = [NSString stringWithFormat:@"%%%d\$\@", i];
            i++;
            if ([format rangeOfString:f].location == NSNotFound) continue;
            else [newArgs addObject:arg];
        }
        va_end(args);

        char *newArgList = (char *)malloc(sizeof(id) * [newArgs count]);
        [newArgs getObjects:(id *)newArgList];
        NSString* result = [[[NSString alloc] initWithFormat:format arguments:newArgList] autorelease];
        free(newArgList);
        return result;
    }
    return nil;
}

The basic algorithm is:

1. search the format string for the %1$@, %2$@ variables by searching for %@
2. if not found, call the normal stringWithFormat and return
3. else, loop over the args
4. if the format has a position variable (%i$@) for position i, add the arg to the new arg array
5. else, don't add the arg
6. take the new arg array, convert it back into a va_list, and call initWithFormat:arguments: to get the correct string.

The idea is that I would run all [NSString stringWithFormat:] calls through this method instead.

This might seem unnecessary to many, but click on to the referenced SO question (first line) to see examples of why I need to do this.

Ideas? Thoughts? Better implementations? Better Solutions?

Answer 1

Whoa there!

Instead of screwing with a core method that you very probably will introduce subtle bugs into, instead just turn on "Static Analyzer" in your project options, and it will run every build - if you get the arguments wrong it will issue a compiler warning for you.

I appreciate your desire to make the application more robust but I think it very likely that re-writing this method will more likely break your application than save it.

Answer 2

How about defining your own interim method instead of using format specifiers and stringWithFormat:? For example, you could define your own method replaceIndexPoints: to look for ($1) instead of %1$@. You would then format your string and insert translated replacements independently. This method could also take an array of strings, with NSNull or empty strings at the indexes that don't exist in the “untranslated” string.

Your method could look like this (if it were a category method for NSMutableString):

- (void) replaceIndexPointsWithStrings:(NSArray *) replacements
{
    // 1. look for largest index in "self".
    // 2. loop from the beginning to the largest index, replacing each
    //    index with corresponding string from replacements array.
}

Here's a few issues that I see with your current implementation (at a glance):

1. The __VA_ARGS__ thingy explained in the comments.
2. When you use while (arg = va_arg(args, id)), you are assuming that the arguments are nil terminated (such as for arrayWithObjects:), but with stringWithFormat: this is not a requirement.
3. I don't think you're required to escape the $ and @ in your string format in your arg-loop.
4. I'm not sure this would work well if uaStringWithFormat: was passed something larger than a pointer (i.e. long long if pointers are 32-bit). This may only be an issue if your translations also require inserting unlocalised numbers of long long magnitude.