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: ?

Answer 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.

Answer 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>:

Answer 3

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

Answer 4

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");
}

Answer 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