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: ?

score 107 · Accepted Answer · edited May 05 '13 at 11:15

107

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.

edited May 05 '13 at 11:15

Pang

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answered Aug 18 '09 at 10:41

Abizern

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If you believe Aaron Hillegass then there is no performance difference, only a bit of type safty: http://blog.bignerdranch.com/334-isequal-vs-isequaltostring/ – Caro Dec 25 '13 at 18:47
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Thanks for the link - useful. Although you're asking us to believe Mark Dalrymple - who I do :) – Abizern Jan 11 '14 at 17:24
Updated link: https://www.bignerdranch.com/blog/isequal-vs-isequaltostring/ – Max Mar 05 '19 at 21:35

score 17 · Answer 2 · answered Aug 18 '09 at 16:11

17

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>:

answered Aug 18 '09 at 16:11

Jonathan Dann

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I hadn't come across this before, any documentation that you know of? – Mike Abdullah Aug 19 '09 at 11:21
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This doesn't seem to be true for isEqualToString, which just returns NO if you pass in nil. – Jaka Jančar Aug 21 '09 at 09:48
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Interesting, it's documented in the Object Comparison section of the Cocoa Fundamentals Guide – Jonathan Dann Aug 23 '09 at 16:56
This is not true. isEqualToString does not raise an exception. – respectTheCode Jun 19 '13 at 16:55
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The Cocoa Fundamentals Guide web page says, "This document may not represent best practices for current development." It's old, apparently. – cbh2000 Dec 24 '13 at 20:56

respectTheCode · Answer 3 · 2013-06-19T16:58:25.523

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");
}

score 5 · Answer 4 · answered Aug 18 '09 at 09:38

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

answered Aug 18 '09 at 09:38

iKenndac

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Your guess is probably true:) – Philip007 Jan 17 '13 at 07:53

Ben Packard · Answer 5 · 2018-07-19T13:21:40.290

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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

edited Jul 19 '18 at 13:21

answered Aug 03 '13 at 13:39

Ben Packard

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I don't see any difference in the way nil is handled by the two. Be nil be the receiver or argument or both. – SayeedHussain Dec 13 '14 at 05:07
Whatever "this" is no longer exists :/ – Jared Grubb Jul 18 '18 at 21:53
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Thanks @JaredGrubb, I found the new URL. – Ben Packard Jul 19 '18 at 13:22

NSString: isEqual vs. isEqualToString

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