NSString 小问题(运行时)

这段代码会有什么问题?

  NSString *string = [[NSMutableArray alloc]init];
  [string stringByAppendingString:@"abc"];
  [string addObject:@"abc"];

1.字符串类型指针指向数组, 编译器警告⚠️

Incompatible pointer types initializing 'NSString *' with an expression of type 'NSMutableArray *'

2.string运行时为可变数组, 没有stringByAppendingString方法 (去掉第三行运行) 直接崩溃

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayM stringByAppendingString:]: unrecognized selector sent to instance 0x60000309d9b0'

3.string调用addObject 方法, 编译期为字符串类型, 不用运行编译期直接报错 ❌

No visible @interface for 'NSString' declares the selector 'addObject:'