Why isn't NSString answering to call by reference? - objective-c

Question

I'm trying to change a variable of NSString type by calling a method. But it doesn't work. I tried this very same method with othe types and it worked perfectly fine. Can you help me out there?
Interface:
- (void) changeNSString: (NSString *) stringToChange;
Implementation:
- (void) changeNSString: (NSString *) stringToChange{
stringToChange = #"Test";
}
- (IBAction)actionBla:(id)sender {
NSString *myString = [NSString string];
[self changeNSString:myString];
NSLog(myString); //Returns nothing or empty string
}
======================
Edit: thanks to the help of CrimsonDiego, here is a working solution. Though I still have a little understanding issue: why is there no type-error when assigning a NSMutableString to a NSString var?
Interface:
- (void) changeNSMutableString: (NSMutableString *) stringToChange;
Implementation:
- (void) changeNSMutableString: (NSMutableString *) stringToChange{
[stringToChange setString:#"Test"]
}
- (IBAction)actionBla:(id)sender {
NSMutableString *myString = [NSMutableString string];
[self changeNSMutableString:myString];
self.myLabel.text = myString; //Why is there no type error???
}

Answer 1

You are assigning a new variable to a local variable within the function.
The local variable stringToChange references the same variable as myString at first, but when you assign #"Test" to stringToChange, it no longer references the original string.
The solution here would be to use NSMutableString, or have a class variable (or property) and use that instead of passing it to the function.

Answer 2

Your setting stringToChange as local in your function. Try the following:
- (NSString*) changeNSString: (NSString *) stringToChange{
stringToChange = #"Test";
return stringToChange;
}
- (IBAction)actionBla:(id)sender {
NSString *myString = [NSString string];
myString = [self changeNSString:myString];
NSLog(myString); //Returns nothing or empty string
}

Answer 3

I am no expert on Objective-C but it seems to me that stringToChange, as in "normal" C, is a parameter that can contain a pointer to a string. The parameter is pass-by-value and thus, when you assign #"Test" to stringToChange you are actually only changing the local parameter stringToChange and not myString. If you want to change myString you have to change the parameter type to NSString ** and pass &myString as a parameter instead.