Simulate static variables and static class inheritance

In objective-C, there is no static class variable, but we can simulate it. However, if there is an inheritance relationship between class objects, this is a little complicated. Let's assume that a coupon class object needs to save the number of coupons currently remaining, and a newcoupon Class Object inherited from the coupon class object, but it also saves the number of available coupons, the following example shows how to ensure that their static class variables are independent of each other.

Coupon. h

 
# Import<Foundation/Foundation. h>

@ Interface coupon: nsobject {

}

+(Int) Numbercouponsleft;
+(Void) Resetcoupon;

@ End
 

Coupon. m

# Import  "  Coupon. h  "  
  # Define  Initialize al_coupon_allocation 100  

  //  Set static variables  
  Static     Int  Availablecoupons  = Initial_coupon_allocation;

  //  Define Private Static accessors in implementation classes  
  @ Interface coupon ()
  +  (  Int  ) Availablecoupons;
  +  (  Void  ) Setavailablecoupons :(  Int  ) Inavailablecoupons;
@ End

@ Implementation coupon

 +  (  Int  ) Availablecoupons
{
  Return  Availablecoupons;
}

  +  (  Void  ) Setavailablecoupons :(  Int  ) Inavailablecoupons
{
Availablecoupons  =  Inavailablecoupons;
}

  // To support inheritance of static classes, focus on init.  
  -  (ID) Init
{
  If  (Self  =  [Super init])
{
  //  [Self class] indicates the object used as the receiver. If it is an inherited class, it refers to the inherited class object.  
    If  ([Self  Class  ] Availablecoupons] =     0  )
{
  //  If the valid coupon value is 0, this class object is destroyed.  
  [Self release];
  Return  Nil;
}

  //  Currently valid coupon minus 1  
  [[Self  Class  ] Setavailablecoupons: [[self Class  ] Availablecoupons]  -  1  ];
}
  Return  Self;
}

  +  (  Int  ) Numbercouponsleft
{
  Return  [Self availablecoupons];
}

  +  (  Void ) Resetcoupon
{
[Self setavailablecoupons: initial_coupon_allocation];
}

@ End 

Newcoupon. h

 

# Import<Foundation/Foundation. h>
# Import"Coupon. h"


@ Interface newcoupon: coupon {

}

@ End
 

Newcoupon. m

 

 # Import  "  Newcoupon. h  "  
  # Define Initial_new_coupon_allocation 200  

  //  Set new static variables in the inheritance class  
  Static     Int  Availablecoupons  =  Initial_new_coupon_allocation;

@ Interface newcoupon ()
  +  (  Int  ) Availablecoupons;
 +  (  Void  ) Setavailablecoupons :(  Int  ) Inavailablecoupons;
@ End

@ Implementation newcoupon

  +  (  Int  ) Availablecoupons
{
  Return  Availablecoupons;
}

  +  (  Void ) Setavailablecoupons :(  Int  ) Inavailablecoupons
{
Availablecoupons  =  Inavailablecoupons;
}

  +  (  Void  ) Resetcoupon
{
[Self setavailablecoupons: initial_new_coupon_allocation];
}

@ End 

Main. m

 # Import  <  Foundation  / Foundation. h  >  
# Import  "  Coupon. h  "  
# Import  "  Newcoupon. h  "  

  Int  Main (  Int  Argc,  Const    Char     *  Argv [])
{

NSAID utoreleasepool  *  Pool  =  [[NSAID utoreleasepool alloc] init];

  //  Insert code here...
  //  Nslog (@ "Hello, world! ");  
  
[[Coupon alloc] init];
Nslog (  @" % D  "  , [Coupon numbercouponsleft]);
[Coupon resetcoupon];
Nslog (  @"  % D  "  , [Coupon numbercouponsleft]);

[[Newcoupon alloc] init];
Nslog (  @"  % D  "  , [Newcoupon numbercouponsleft]);
[Newcoupon resetcoupon];
Nslog (  @"  % D  " , [Newcoupon numbercouponsleft]);

[Pool drain];
  Return     0  ;
} 

Returned results:

 [Switching to process  3200  Thread  0x0  ]
  2011  -  05  -  03    10  :  15  :  08.505  Demo04 [  3200  :  903  ]  99  
  2011  -  05  -  03    10  :  15  :  08.508  Demo04 [  3200  :  903  ]  100  
  2011  -  05  -  03    10  :  15  :  08.508  Demo04 [  3200  :  903  ]  199  
  2011  -  05  -  03    10  :  15  :  08.509  Demo04 [  3200  :  903  ]  200