Explaination of "warning: assignment from distinct Objective-C type"

Question

If I set a mutable string's value to a value from an array, and use the following code to manipulate it:

NSMutableString *theCountry = [listItems objectAtIndex:3];
theCountry = [theCountry stringByReplacingOccurrencesOfString:@"\"" withString:@""];

I receive the warning "warning: assignment from distinct Objective-C type" after the second line of the above code. If I do not have "theCountry =" before the method call, the warning goes away, but the string does not get manipulated...

Answer 1

The stringByReplacingOccurrencesOfString:withString: method is declared to return an NSString*, not an NSMutableString*. Basically, in the assignment, you're assigning an NSString* to a variable of type NSMutableString* which is not necessarily safe (note that NSMutableString inherits NSString, not the other way around).