Question

NSScanner takes a pointer to an NSString:

NSMutableString *myString;
...
[scanner scanUpToCharactersFromSet:charSet intoString:&myString];
[myString appendString:@"hello"];

But myString is mutable, so I get an error when I try to do an append later, saying I’m trying to mutate an un-mutatable.
Do I have to create temp copies and go back&forth, or is there a more efficient way to do this?

Answer 1

Either do

NSString *myString; //this is just a variable identifier, no object was created and assigned to it yet
...
//scanner will create a NSString and write it to myString. It won't know it is passed to a variable typed NSMutableString or NSString
[scanner scanUpToCharactersFromSet:charSet intoString:&myString];

//instead of creating mutable string you can create a new immutable by appending a string
myString = [myString stringByAppendingString:@"hello"];

or _{better: see the next code snippet}

NSMutableString *myString; //this is just a variable identifier, no object was created and assigned to it yet
...

//scanner will create a NSString and write it to myString. It won't know it is passed to a variable typed NSMutableString
[scanner scanUpToCharactersFromSet:charSet intoString:&myString];
//myString contains a NSString now, not NSMutableString

myString = [myString mutableCopy]; // make a mutable copy and assign it to the variable
[myString appendString:@"hello"];

---

In the second approach you will have the inconsistency that for a short while a NSMutable typed variable will hold a immutable string. tis is definitely a flaw. Solution one is much cleaner. If you favor two you should introduce a second NSString variable and pass that to the scanner an d than mutable copy that to your NSMutableString.

NSString *tempString;
NSMutableString *myString; 
...

[scanner scanUpToCharactersFromSet:charSet intoString:&tempString];

myString = [tempString mutableCopy];
[myString appendString:@"hello"];