const

NSString *const 和 const NSString * 的区别

• NSString *const 变量存储的指针可变，变量存储的值不可变

    //A modifiable pointer to a constantNSString (its value can't be modified)
    const NSString * str = @"11";
    str = @"22";
  

• const NSString * 变量存储的值可变，变量存储的指针不可变

    //A constant pointer (not modifiable) to an NSString (its value can be modified)
    NSString *const str1 = @"33";
    str1 = @"44";   //会报错