Question

 int main(int argc, char *argv[]) {         
    @autoreleasepool {
      const int x = 1;
      const NSMutableArray *array1 = [NSMutableArray array];
      const NSMutableString *str1 = @"1";
      NSString * const str2 = @"2";

      // x = 2; compile error
      [array1 addObject:@"2"]; // ok 
      // [str1 appendString:@"2"]; // runtime error
      // Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with appendString:'
      // str2 = @"3"; compile error
     }
 }

my Question is Why array1 addObject is legal and why str1 appendString is forbidden?

see this blow:

NSMutableString *f2(const NSMutableString * const x) {
  [x appendString: @" world!"];
  return x;
}

int main(int argc, char *argv[]) {
    @autoreleasepool {
      NSMutableString *x = [@"Hello" mutableCopy];
      NSLog(@"%@", f2(x));
    }
    return 0;
}

and why this code is legal, how can I make an object immutable using ‘const’ keyword like in c++?

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@see https://softwareengineering.stackexchange.com/questions/151463/do-people-use-const-a-lot-when-programming-in-objective-c

Answer 1

‘const’ does nothing on Objective-C objects. You cannot do what you’re asking for.