[Sintaxis] Asegurarse de que hay sólo 1 instancia de clase

[Kuk]

Ejemplo de Eslopes de una única instancia de clase:

❞ Singleton In traditional Cobol programming all programs are singletons. Well, this a sort of joke, but pretty much basically a procedural program cannot be instantiated. In the other hand OO Cobol classes are by nature just a model to create one, or two, or one billion objects. So, what if our application needs to deal with a single instance of a given class, ensuring that only one instance would exist at anytime? Here comes the GOF Singleton pattern! As we can see in the diagram above, the class constructor is private, so no one can instantiate that class direclty. To get an instance we need to call the method "GetInstance". That method would check if the private attribute called "instance" is null. If so, the method would then call the class constructor NEW (which is visible for it), set the class attribute "instance" with the new instance, and return the instance to the caller. Next time that method is called it would return the already created class instance. Too confuse? I thought so

Step-by-step
1.Program (or some object) invokes Singleton.GetInstance static method
2.GetInstance checks if private attribute contains null or a valid Singleton instance
3.If null, GetInstance calls the class constructor "NEW" to create a new instance
4.The new instance is stored in the private attribute "instance" and returned
5.If not null, the instance already created would be returned
6.There can be only ONE

 CLASS-ID. Singleton AS "CoolThings.Singleton".
     *> Static data and methods
     static.
         data division.
            working-storage section.
            01 instance usage object reference SINGLETON is private.
         
         procedure division.
             method-id. GetInstance AS "GetInstance".
                  data division.
                      linkage section.
                      01 localSingleton usage object reference SINGLETON.
                      
                  procedure division returning localSingleton.
                      if (instance equal null) then
                            invoke Singleton "NEW" returning localSingleton
                            set    instance  to localSingleton
                      else
                            set    localSingleton    to instance
                      end-if
                      
              end method GetInstance.
      end static.
      *> Instance's data and methods       
      object.
          environment division.
              data division.
                  working-storage section.
                  01   counter pic s9(04) value zeros.
          
          procedure division.
              method-id. NEW is private.
                  data division.
                  procedure division.
                      move zeros   to counter.
                  
              end method NEW.
              
              method-id. GetCounter as "GetCounter".
                  data division.
                      linkage section.
                      01 localCounter  pic s9(04).
                          
              procedure division returning localCounter.
                 add   1       to counter.
                
                 move  counter    to  localCounter.
                  
              end method GetCounter.
              
      end object.        
  END CLASS Singleton.

Testing our singleton

program-id. usingSingleton as "usingSingleton".
 
environment division.
     configuration section.
         repository.
             class ClassSingleton as "CoolThings.Singleton".
 
 data division.
     working-storage section.
         01  singletonInstance1  usage object reference ClassSingleton.
         01  singletonInstance2   usage object reference ClassSingleton.
 
 procedure division.
         invoke ClassSingleton "GetInstance" returning singletonInstance1
         invoke ClassSingleton "GetInstance" returning singletonInstance2
         invoke ClassSingleton "GetInstance" returning singletonInstance3
 
         display singletonInstance1::"GetCounter"
         display singletonInstance2::"GetCounter"
         display singletonInstance3::"GetCounter"

Running the code above we would see the following output in the console window.

 0001
 0002
 0003

❞ In our example, the GetCounter method actually updates the counter and return its new value. First call to GetInstance forces the Singleton to be created and our counter to be initialized with zeros, incremented by 1 and returned. Subsequent calls just increase counter value. Notice that we are not creating the Singleton instance in the code above. That's impossible because the constructor (NEW) is private. All that we can do is to request the instance using the "GetInstance" method. Where to use Singletons? You name it! A class that controls the number of database connections? A round robin class that forward the request to a server in a cluster? Overall settings for the application? Possibilities are endless! Download the source Cool things with OO Cobol - Source Code Some references in the web Singleton .NET Design Pattern in C# and VB - dofactory.com Singleton pattern - Wikipedia