why I can not catch the NSInvalidArgumentException in iphone os?

Question

 @try {
  NSString *a = [NSString string];
  [a ggg];
 }
 @catch (NSException * e) {
  self.user_name = nil;
 }

why I can not catch the NSInvalidArgumentException in iphone os?

Answer 1

 @try {
  NSString *a = [NSString string];
  [a ggg];
 }
 @catch (NSException * e) {
 }

why I can not catch the NSInvalidArgumentException in iphone os?