Confusing basic concepts member variables local variables global variables

Member variables

@interface Person:NSObject
{
 int age;
} 
@end


• Variables written in braces of class declarations , We call it a member variable （ attribute 、 Instance variables ）

• Member variables can only be accessed through objects

• Member variables cannot leave the class , After leaving the class, it is not a member variable

• Member variables cannot be initialized while being defined

• Storage ： Pile up （ The storage space of the heap corresponding to the current object ）. Data stored in the heap , Will not be released by the system , Only programmers can release

local variable

-(void)info{
 int age = 0;
}


• Variables written in functions or code blocks , We call it a local variable

• Scope ： Start with the line of definition , Until braces or
  return

• Local variables can be defined first and then initialized , It can also be defined and initialized at the same time

• Storage ： Stack . Data stored in the stack , The system will automatically release

Global variables

@implementation Person
int age = 0;
-(void)info{

}
@end


• Variables written outside functions and braces , It's called all variables

• Scope ： Start with the line of definition , All the way to the end of the file

• Global variables can be defined first and then initialized , It can also be defined and initialized at the same time

• Storage ： Static zone , As soon as the program starts, it allocates storage space , Not released until the end of the program

• There are two types of global variables ：

• static
  Modify global variable , Available only in this source file

 static int age =3;
 @implementation Person
 @end

 -  stay OC Declared in the syntax of `static` Static variables cannot be accessed directly through class names in other classes , Its scope can only be in the declared .m In file  . However, you can call the method of this class to indirectly modify the value of this static variable
 - `static` The variables declared by the keyword must be placed in `implementation` outside , Or in the method , If you don't assign a value to it , The default is 0, It initializes only once the program starts up （ It is not initialized after the class is instantiated ）
 -  take static Static variables are written in methods , Its initialization is also when the program starts up , Once the program starts `static` Cannot be created in


• extern
  Modify global variable , Can be referenced by other classes .h The header file states

 //.h In file
 extern int age;

 .m Implement the assignment in the file

 //.m In file
 #import "Person.h"
 int age = 10;
 @implementation Person
 
 @end 


• extern
  Can be placed before variables or functions , To represent the definition of a variable or function in another file , Prompt the compiler when it encounters this variable or function , Look for its definition in other modules . in addition ,
  extern
  It can also be used to specify Links

• Be careful ： Naming must be unique , To distinguish from global variables in other source files , If a global variable with the same name as another source file appears , It will report an error

• 

• difference ：

• extern
  Decorated global variables have external links by default , Scope is the whole project , Global variables defined in a file , In another file , adopt external Declaration of global variables , You can use global variables .

• static
  Decorated global static variables , The scope is the file where the variable is declared .