Question

I want to do the following:

["Hello <firstname> <middlename> <lastname>". replaceTokensWithStrings: 
  @"firstname", someFirstName, 
  @"middlename", middleNameMightBeNilObject,
  @"lastname", lastNameObject];

It looks like this is not possible because of the “nil as a terminator problem”.

However NSLog also takes multiple arguments and can handle nil as parameters too:

NSString *nilValue = nil;

NSLog(@"Value of nilValue=%@", nilValue);

output

Value of nilValue=(null)

So how can I achieve this in my replaceTokensWithStrings:... method?

Update: The signature of my method:

-(NSString *)replaceTokensWithStrings:(NSString *)input, ... NS_SOMETHING_SPECIAL_HERE{

Answer 1

NSLog counts the number of format specifiers (%@, %i, etc.) in its format string (the first argument, the literal string) in order to know how many arguments it should pull off the call stack. This is why it can handle nil arguments — it is using the count to terminate its processing (and, incidentally, why it will often crash if you give it too few arguments).

Since you have a format-type string, you can do the same thing — just whip up a helper method that counts the number of <something> elements in the string on which replaceTokensWithStrings: has been called, and use the results to limit your processing of the va_list.

The NS_SOMETHING_SPECIAL in your method declaration would be NS_REQUIRES_NIL_TERMINATION for the case where you’re using nil as a sentinel. You don’t need anything special if you’re getting the count of arguments from somewhere.