Question

If I need to compare a NSString property with a constant string define by #define, can I use double equal sign?

I know that isEqualToString: would work, but just wonder if == also works, say for the following two scenarios:

Scenario 1:

#define BLA @"BLA"
NSString *str1 = BLA;
BOOL equal = self.someStr == BLA;

Scenario 2:

#define BLA @"BLA"
NSString *str1 = @"BLA";
BOOL equal = self.someStr == BLA;

are they the same?

Answer 1

== and isEqualToString: are never the same. While == may sometimes behave as if it is checking the actual characters in a string, it is completely by chance. The LLVM compiler heavily optimizes string constants however LLVM optimizations are an implementation detail and are subject to change at any time.

isEqualToString: – compares individual characters in a string.
== – straight up pointer comparison. This only returns true when both NSString objects are actually the same instance at the same exact memory location.

Edit: #define statements are evaluated before the compiler runs (they are evaluated in the preprocessor). #define statements do a straight up search and replace so putting the same thing instead of a #define is the same exact thing.