How to use an NSMutableString in a method that takes an NSString?

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?

vikingosegundo · Accepted Answer · 2013-02-09T15:56:57.740

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"];

Your second example is a bit funny, because after the `[scanner ...` line, you have a `NSMutableString` variable with an immutable string in it. I think your first one is much better. — Jesse Rusak, Feb 09 '13 at 15:52
@JesseRusak, I changed my answer to reflect that more clearly. — vikingosegundo, Feb 09 '13 at 15:58

score 1 · Answer 2 · answered Feb 09 '13 at 15:49

In case you need to repeat that multiple times, do it with only one mutable string:

NSMutableString *mutableString = [NSMutableString string];
NSString *string = nil;

[scanner scanUpToCharactersFromSet:charSet1 intoString:&string];
[mutableString appendString:string];

[scanner scanUpToCharactersFromSet:charSet2 intoString:&string];
[mutableString appendString:string];

//...

No need to copy them or do any other conversion.

How to use an NSMutableString in a method that takes an NSString?

2 Answers2