When would initWithFormat:arguments: be used?

Question

The NSString method initWithFormat:arguments: takes a va_list as an argument. I can't figure out when it would be used (or even how to use it). Why would Apple add such a method when the regular initWithFormat: is much more user-friendly?

Answer 1

I would say without looking further into it that Apple provide NSString initWithFormat: as a utility method on top of NSString initWithFormat:arguements: meaning the short version just ends up calling the longer one.

There's also [NSString stringWithFormat:] that can return an autoreleased NSString, saving you the alloc call if you don't need the string around for long.

Answer 2

It's useful when your own function or method uses variadic arguments, because in that case it is impossible to use the vanilla initWithFormat: method.

For instance, the following (useless) example snippet:

void log(NSString* format, ...)
{
    va_list arguments;
    va_start(list, format);

    // impossible:
    // NSString* formattedString = [[NSString alloc] initWithFormat: ???];

    // possible
    va_list argsCopy;
    va_copy(list);
    NSString* formattedString = [[NSString alloc] initWithFormat:format arguments:argsCopy];    

    // do something cool with your string
    NSLog(@"%@", formattedString);
    va_end(argsCopy);
    va_end(list);
}

Answer 3

You can't pass a dynamic list of format arguments to -initWithFormat:. For example, if you wanted to implement -stringByAppendingFormat: yourself without -initWithFormat:arguments:, you'd have a job of it. With the va_list version, you could do it:

- (NSString *)stringByAppendingFormat:(NSString *)format, ... {
    va_list args;
    va_start(args, format);
    NSString * result = [self stringByAppendingString:[NSString stringWithFormat:format arguments:args]];
    va_end(args);
    return result;
}