In Objective-C, how do you initialise a constant from another constant?

Question

 NSString* const nits = @"nits";
 NSString* const nuts = nits;  // error: "initializer element is not constant"

How is this done?

Answer 1

I don't know objective-c but I would imagine that by initialising something from a value that is not a literal then you end up with not a constant - regardless of the initial value declaration.

Answer 2

Constant String literals such as your @"nits" are hardcoded into objective-c binary files. String constants such as nits and nuts have to be defined as a constant string literal, as they are hard-coded as well. The assignment nuts = nits does not work because nits is not a constant string literal, even though the variable is constant at runtime.

There are two ways to fix this:

1. write the string literal @"nits" twice
2. use a #define

This might look like this:

 #define NitsNutsString @"nits"
 NSString* const nits = NitsNutsString;
 NSString* const nuts = NitsNutsString;

Alternatively you can use #defines only, but these cannot be exported in a header as cleanly as const strings.