"sending 'const NSString *' to parameter of type 'NSString *' discards qualifiers" warning

Question

I have Constants NSString, that I want to call like:

[newString isEqualToString:CONSTANT_STRING];

Any wrong code here?

I got this warning:

sending 'const NSString *' to parameter of type 'NSString *' discards qualifiers

How should these be declared?

Answer 1

You should declare your constant string as follows:

NSString * const kSomeConstantString = @""; // constant pointer

instead of:

const NSString * kSomeConstantString = @""; // pointer to constant
// equivalent to
NSString const * kSomeConstantString = @"";

The former is a constant pointer to an NSString object, while the latter is a pointer to a constant NSString object.

Using a NSString * const prevents you from reassigning kSomeConstantString to point to a different NSString object.

The method isEqualToString: expects an argument of type NSString *. If you pass a pointer to a constant string (const NSString *), you are passing something different than it expects.

Besides, NSString objects are already immutable, so making them const NSString is meaningless.