gistfile1.m

@interface Class : NSObject

extern const struct ClassConstants {
  __unsafe_unretained NSString *foo;
  __unsafe_unretained NSString *bar;
} ClassContstants;

@end

@implementation Class

const struct ClassContstants ClassContstants = {
  .foo = @"kClassConstantsFoo",
  .bar = @"kClassConstantsBar",
};

- (void)someMethods
{
  NSLog(@"%@", ClassConstants.foo); // Prints kClassConstantsFoo
}

@end