Objective C - Pass by value and pass by reference

Question

First of all, sorry for this simple question. But I need to understand what is happening.

I thought the output should be upper case string. But it comes out to be UPPER CASE STRING.

- (void)test {      NSString *stringVar = @"UPPER CASE STRING";      [self changeString:stringVar];      NSLog(@"value after changed : %@", stringVar); }  - (void)changeString:(NSString*)string {      string = [string lowercaseString]; } 

What is happening and how to fix it?

Answer 1

The [string lowercaseString] call creates a new NSString object that you assign to the local variable string. This does not change the value of stringVar outside the changeString function. The pointer itself is passed by value.

One way to do what you want, is to pass a pointer to a pointer:

-(void) test {      NSString *stringVar = @"UPPER CASE STRING";      [self changeString:&stringVar];      NSLog(@"value after changed : %@", stringVar); }  -(void) changeString:(NSString**)string {      *string = [*string lowercaseString]; } 

Answer 2

If you look at the reference to the [NSString lowerCaseString] method, you can see that it returns a new string, with the lowercase'd characters:

Returns lowercased representation of the receiver.

- (NSString *)lowercaseString 

What your code does is simply overwrite the reference to the input string with the output of the lowercaseString call, which has no effect. The best way to solve this issue is for you to return the value yourself, which makes the method easier to understand:

-(void) test {      NSString *stringVar = @"UPPER CASE STRING";      stringVar = [self changeString:stringVar];      NSLog(@"value after changed : %@", stringVar); }  -(NSString *) changeString:(NSString*)string {      string = [string lowercaseString];      return string; } 

You need to understand that NSString is immutable so there is no way, other than reassigning the reference, to change a string's contents. You could, however use NSMutableString instead, which can be modified in place.