How to create constant NSString by concatenating strings in Obj-C?

Question

I'm trying to instanciate a constant NSString by concatanating other NSString instances.

Here is what I'm doing in my implementation file :

static NSString *const MY_CONST = @"TEST";
static NSString *const MY_CONCATENATE_CONST = [NSString stringWithFormat:@"STRING %@", MY_CONST];

It leads to the following compilation error : Initializer element is not constant

I suppose this is because stringWithFormat doesn't return a constant NSString, but since there is no other way to concatenate strings in Obj-C, what am I supposed to do ?

Thanks for your help,

Eric.

Answer 1

I thought there must be a way to do this but the best I could do was using a #define directive. For example,

// Define the base url as an NSString
#define BASE_URL @"http://www.milhouse.co.uk/"

// Now the derived strings glued by magic
NSString *const kBaseURL    = BASE_URL;
NSString *const kStatusURL  = BASE_URL @"status.html";
NSString *const kBalanceURL = BASE_URL @"balance.html";

Answer 2

static const objects value is determined at compile-time so you indeed cannot add any method calls to their initialization. As an alternative you can do the following:

static NSString *const MY_CONST = @"TEST";
static NSString *MY_CONCATENATE_CONST = nil;

if (nil == MY_CONCATENATE_CONST)
   MY_CONCATENATE_CONST = [NSString stringWithFormat:@"STRING %@", MY_CONST];