Question

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Understanding NSString comparison in Objective-C

Was just reading up about equality vs identity and I realized that I’ve been using some equal signs when comparing strings in my objc code. The weird thing is that it actually works from times to times and I was wondering why.

http://www.karlkraft.com/index.php/2008/01/07/equality-vs-identity/

I have two pieces of code, one work and one doesn’t.

WORKING. Here I have a object called ‘Category’ which has a NSString property called ‘name’.

@property (nonatomic, retain) NSString *name;

I then have a function where I pass in a ‘Catergory’ pointer and when I use the equal sign here, it works perfectly.

 -(void)addCategoryToStorage:(Category*)newcategory {
      if(newcategory.name != @"All") {  // this works

NON WORKING. Here I use the output from a NSFileManager call;

NSArray *dirContent = [self.fileManager 
                       contentsOfDirectoryAtPath:@"MyFiles"
                       error:nil];

I then try to compare on of the NSStrings in the array:

for(int i = 0; i < [dirContent count]; i++) { 
    if([dirContent objectAtIndex:i] == @"MyFile") // This will never be True

So my questions is, what would be the reason why these two NSString pointers would work so differently? I know I shouldn’t use “==” but at the moment, but I’m just so curious why it works sometimes.

I noticed some difference between them debugging. In the first case, the “newcategory”s “name” said (__NSCFConstantString*) and had the value visible.
For some reason, the “dirContent” arrays contains two (__NSString*) objects where the values aren’t visible.

This raises another question. What makes the NSString in the first case to become a constant? It seems to have something to with how it’s created when I pass it to the function. When I allocate a new object with a NSString property that I then pass in to a function, the function sees it a constant(The NSString property of the object). But when the object is stored in a list and I grab it by passing [array objectAtIndex:index] into a function, the functions doesn’t get it as a constant (the NSString property of the object that is).

Thanks!

Answer 1

It may work, when the pointers point to the same object.

If you have a string literal in your code, it is actually a statically allocated object of type NSString. At any point in your code, if you would reference this string value, you are actually using the same pointer. (A pointer is basically a memory address)

If you have the string literal @"abc" at more than one point in your code, the compiler only puts in one instance, and uses a pointer to this single instance.

This explains why @"abc" == @"abc".

Taking your example:

if(newcategory.name != @"All")

will only work if at some point in your code you did:

newcategory.name = @"All";

And will not work if you would do:

newcategory.name = [NSString stringWithFormat:@"%c%c%c",'A','l','l'];

because in that last case you explicitly allocate a new string.

edit just tested, this example was flawed:

newcategory.name = [NSString stringWithString:@"All"];

because this is optimized away so you will have the same pointer to @"All"