About static type, dynamic type, ID, InstanceType

The static type

• When a pointer variable is defined as a specific object, it is statically typed. The class to which the pointer variable belongs is known at compile time. This variable always stores objects of that particular class

  @interface Person : NSObject property (nonatomic,strong) NSString *name; - (void)run;
  @end
  @implementation Person
  -(void)run{
      NSLog(@"run");
  }
  @end
  int main(int argc, const char * argv[]) {
      Person *p = [[Person alloc]init];
      p.name = @"ABC";
      [p run];
      return 0;
  }
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• Characteristics of static data types:

  1. The type of the variable is known at compile time

  We know at compile time that P is of type PersonCopy the code

  2. Know what properties and methods are in a variable

  @property (nonatomic,strong) NSString *name; - (void)run;
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  3. These properties and methods are accessible at compile time

   p.name = @"ABC";
    [p run];
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  4. And if you define variables with static data types, the compiler will report an error if you access properties and methods that are not static data types

  

Dynamic type

• Refers to a program that does not determine which class an object belongs to until execution

  @interface Person : NSObject @property (nonatomic,strong) NSString *name; - (void)run;
  @end
  @implementation Person
  -(void)run{
      NSLog(@"run"); } - (void)test{
      NSLog(@"test"); } @end @interface Student : Person @property (nonatomic,assign) int age; - (void)eat;
  @end
  @implementation Student
  -(void)eat{
      NSLog(@"eat");
  }
  @end
  
  int main(int argc, const char * argv[]) {
      id obj1 = [[Person alloc]init];
      [obj1 run];
      id obj2 = [[Student alloc]init];
      [obj2 eat];
      return 0;
  }
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• Characteristics of dynamic data types:

1. The compiler does not know the true type of a variable at compile time, but only at run time

   id obj1 = [[Person alloc]init];
   id obj2 = [[Student alloc]init];
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idThe type andinstancetype

• idtype
  • idIs a generic object type that can point to objects belonging to any class. It can be understood as a universal pointervoid*Data type,idIt is also a dynamic data type that can be used to define variables, as function arguments, as function return values, and so on
    • id == NSObject*(Universal pointer)
    • NSObject*Is a static type
  • Because dynamic data types can call any method, it is possible to call a method that is not their own without reporting an error at compile time, resulting in a runtime error
  • Usage scenario: Polymorphic, which reduces the amount of code required to call subclass-specific methods and avoids mandatory type conversions
  • The ID type cannot use dot syntax because dot syntax is a compiler property, while the ID type is a runtime property
  • Use static typing as much as possible. Static typing can catch errors earlier and improve readability
• instancetype
  • instancetypeThe true type of an object can be determined at compile time
  • instancetypeCan only be used for return values
  • Custom constructor, return value as much as possibleinstancetype, do not useid