NSString: isEqual vs. isEqualToString

Question

What is the difference between isEqual: and isEqualToString:?

Why are classes adding isEqualTo* methods (isEqualToArray for NSArray, isEqualToData for NSData, ...) instead of just overriding isEqual: ?

Solution 1

isEqual: compares a string to an object, and will return NO if the object is not a string. isEqualToString: is faster if you know both objects are strings, as the documentation states:

Special Considerations

When you know both objects are strings, this method is a faster way to check equality than isEqual:.

isEqualTo<Class> is used to provide specific checks for equality. For instance; isEqualToArray: checks that the arrays contain an equal number of objects, and that the objects at a given index return YES for the isEqual: test.

Solution 2

Also, for writing your own -isEqual: and -isEqualTo<Class>: methods, the convention is to allow nil arguments for -isEqual: and raise an exception for nil arguments to -isEqualTo<Class>:

Solution 3

Expanding on @Abizern and @Jonathan Dann answers, both isEqual and isEqualToString work with nil values.

- (void)testStringEqual {
    NSString *string = nil;

    STAssertFalse([string isEqual:@"test"], @"NSString isEqual");
    STAssertFalse([string isEqualToString:@"test"], @"NSString isEqualToString");

    // Note that these both return NO
    STAssertFalse([string isEqual:nil], @"NSString isEqual");
    STAssertFalse([string isEqualToString:nil], @"NSString isEqualToString");

    string = @"test";

    STAssertTrue([string isEqual:@"test"], @"NSString isEqual");
    STAssertTrue([string isEqualToString:@"test"], @"NSString isEqualToString");

    STAssertFalse([string isEqual:nil], @"NSString isEqual");
    STAssertFalse([string isEqualToString:nil], @"NSString isEqualToString");
}

Solution 4

My guess is that it provides a slight performance enhancement, as isEqualToString: won't have to type-check what's passed in.

Solution 5

I highly recommend this. The performance benefits of isEqualToString are basically negligible for most applications. But there are two other distinctions the author mentions:

• Type safety
• The way nil is handled

View more solutions

Comments

[Mike Abdullah]

I hadn't come across this before, any documentation that you know of?

[Jaka Jančar]

This doesn't seem to be true for isEqualToString, which just returns NO if you pass in nil.

[Jonathan Dann]

Interesting, it's documented in the Object Comparison section of the <a href="developer.apple.com/documentation/Cocoa/Conceptual/… Fundamentals Guide</a>

[Philip007]

Your guess is probably true:)

[respectTheCode]

This is not true. isEqualToString does not raise an exception.

[cbh2000]

The Cocoa Fundamentals Guide web page says, "This document may not represent best practices for current development." It's old, apparently.

[Caro]

If you believe Aaron Hillegass then there is no performance difference, only a bit of type safty: blog.bignerdranch.com/334-isequal-vs-isequaltostring

[Abizern]

Thanks for the link - useful. Although you're asking us to believe Mark Dalrymple - who I do :)

[SayeedHussain]

I don't see any difference in the way nil is handled by the two. Be nil be the receiver or argument or both.

[Jared Grubb]

Whatever "this" is no longer exists :/

[Ben Packard]

Thanks @JaredGrubb, I found the new URL.

[Max]

Updated link: bignerdranch.com/blog/isequal-vs-isequaltostring