Everything in Cocoa is a pointer.
So you'd do something like:
NSMutableString *myStr = [[NSMutableString alloc] init]; (which is the generic format for all objects.)
Then you can use appendFormat: to append stuff in printf-style format:
i.e.
[myStr appendFormat:@"Mooof! %f %@", myFloat, myNSNormalOrMutableString];