Objective-c – Is an NSMutableString in this case more efficient than NSString

cocoa-touchiphoneobjective c

If I declare a pointer variable in this fashion:

NSString *foo;

And then somewhere later in my code do something such as:

foo = @"bar";

Is that ultimately taking my NSString, creating a new copy in memory with an additional string, then deleting my initial string? Would foo be better off as an NSMutableString?

Best Answer

No, foo is variable holding a pointer to an NSString. The assignment foo = @"bar" sets the value stored by the pointer foo to the address of the NSString @"bar". There is no copy made. If foo already pointed to an other NSString instance that was not a string constsant (i.e. like @"bar") and there are no other references to that instance, then you have a memory leak. You would

[foo release];
foo = @"bar";

in that case. You do not need to retain or release string constants like @"bar".

String constants cannot be mutated, so you will get a runtime error if you try to modify the value of a constant string. There's no difference between assigning @"bar" to an NSString* vs an NSMutableString*. Of course, you won't be able to use the mutating methods of the NSMutableString without a runtime error just because you assign the address of @"bar" (an NSString instance) to a variable of type NSMutableString*. If you want to mutate the string, you would do

NSMutableString *mutableFoo = [@"bar" mutableCopy];

In this case, a copy is obviously made and you are responsible for releasing mutableFoo when you're done with it.

Related Solutions

Iphone – Datatypes for use with NSXMLParser

I don't know where your code is failing, but here's the correct way to handle this:

NSMutableString *buffer = [[NSMutableString alloc] init];

- (void)parser:(NSXMLParser *)parser didStartElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qualifiedName attributes:(NSDictionary *)attributeDict {
    [buffer setString:@""];
}

- (void)parser:(NSXMLParser *)parser didEndElement:(NSString *)elementName namespaceURI:(NSString *)namespaceURI qualifiedName:(NSString *)qName {
    if([elementName isEqualToString:@"number"]) {
        [aChild setNumber:[buffer intValue]];
        [buffer setString:@""];
    }
}

- (void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string {
    [buffer appendString:string];
}

R – Objective-C switch defines contents of string

Your code is perfectly valid. There's no need to use NSMutableString.

Your code does leak memory, because you're not releasing the empty string you allocated using alloc. It's better to use the "string" class method for empty strings:

NSString *aString;

switch ([self getNumberOfSides]) {
        case 1:
                aString = @"A String"; 
                break;
        case 2:
                aString = @"Another String"; 
                break;
        default:
                aString = [NSString string];
                break;
}

Best Answer

Related Solutions

Iphone – Datatypes for use with NSXMLParser

R – Objective-C switch defines contents of string

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