About static type, dynamic type, ID, instancetype

Static type

• When a pointer variable is defined as a specific object , Using static types , The class of this pointer variable is known at compile time , This variable always stores objects of a specific class , By default, all data types are static

 @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;
 }


• Characteristics of static data types ：

• You know the type of variable at compile time

  You know it at compile time p yes Person type


• Know which properties and methods are in the variable

 @property (nonatomic,strong) NSString *name;
 -(void)run;


• These properties and methods can be accessed at compile time

 p.name = @"ABC";
 [p run];


• And if you define variables through static data types , If you access properties and methods that are not static data types , Then the compiler will report an error

Dynamic type

• It refers to the class that the program does not determine the object belongs to until it is executed

 @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;
 }


• Characteristics of dynamic data types ：

• At compile time, the compiler doesn't know the real type of the variable , The real type of it is known only at run time

 id obj1 = [[Person alloc]init];
 id obj2 = [[Student alloc]init];


id

The type and

instancetype

• id
  type

• id
  Is a common object type , It can point to objects belonging to any class , It can be understood as a universal pointer , amount to C Linguistic
  void*
  data type ,
  id
  It is also a dynamic data type , Can be used to define variables , As a function parameter , Return value as a function, etc

• id == NSObject*
  （ Universal pointer ）

• NSObject*
    It's a static type

• Because dynamic data types can call any method , So it's possible to call methods that don't belong to you , And no error is reported during compilation , So it leads to runtime errors

• Use scenarios ： polymorphic , Can reduce the amount of code , To avoid calling subclass specific methods, you need to force type conversion

• id Type cannot use point syntax , Because point syntax is a feature of the compiler , and id Types are runtime characteristics

• Try to use static types , Static types can detect errors earlier , Improve readability

• instancetype

• instancetype
  The true type of the object can be determined at compile time

• instancetype
  Can only be used for return values

• Custom construction method , Use the return value as much as possible
  instancetype
  , Do not use
  id