Global Singleton

From Rosetta Code

A Global Singleton is a class of which only one instance exists within a program. Any attempt to use non-static members of the class involves performing operations on this one instance.

Contents 1 Ada 1.1 Non Thread Safe 1.2 Thread Safe 2 C++ 2.1 Thread-safe 2.2 Non-Thread-Safe 3 D 4 E 5 Io 6 Objective-C 6.1 Non-Thread-Safe 7 Java 7.1 Thread-safe 7.2 Non-Thread-Safe

[edit] Ada

[edit] Non Thread Safe

package Global_Singleton is
   procedure Set_Data (Value : Integer);
   function Get_Data return Integer;
private
   type Instance_Type is record
      -- Define instance data elements
      Data : Integer := 0;
   end record;
   Instance : Instance_Type;
end Global_Singleton;

package body Global_Singleton is
 
   --------------
   -- Set_Data --
   --------------
 
   procedure Set_Data (Value : Integer) is
   begin
      Instance.Data := Value;
   end Set_Data;
 
   --------------
   -- Get_Data --
   --------------
 
   function Get_Data return Integer is
   begin
      return Instance.Data;
   end Get_Data;
 
end Global_Singleton;

[edit] Thread Safe

package Protected_Singleton is
   procedure Set_Data (Value : Integer);
   function Get_Data return Integer;
private
   protected Instance is
      procedure Set(Value : Integer);
      function Get return Integer;
   private
      Data : Integer := 0;
   end Instance_Type;
end Protected_Singleton;

package body Protected_Singleton is
 
   --------------
   -- Set_Data --
   --------------
 
   procedure Set_Data (Value : Integer) is
   begin
      Instance.Set(Value);
   end Set_Data;
 
   --------------
   -- Get_Data --
   --------------
 
   function Get_Data return Integer is
   begin
      return Instance.Get;
   end Get_Data;
 
   --------------
   -- Instance --
   --------------
 
   protected body Instance is
 
      ---------
      -- Set --
      ---------
 
      procedure Set (Value : Integer) is
      begin
         Data := Value;
      end Set;
 
      ---------
      -- Get --
      ---------
 
      function Get return Integer is
      begin
         return Data;
      end Get;
 
   end Instance;
 
end Protected_Singleton;

[edit] C++

[edit] Thread-safe

Operating System: Microsoft Windows NT/XP/Vista

class Singleton
{
public:
     static Singleton* Instance()
     {
          // We need to ensure that we don't accidentally create two Singletons
          HANDLE hMutex = CreateMutex(NULL, FALSE, "MySingletonMutex");
          WaitForSingleObject(hMutex, INFINITE);

          // Create the instance of the class.
          // Since it's a static variable, if the class has already been created,
          // It won't be created again.
          static Singleton myInstance;

          // Release our mutex so that other application threads can use this function
          ReleaseMutex( hMutex );

          // Free the handle
          CloseHandle( hMutex );

          // Return a pointer to our mutex instance.
          return &myInstance;
     }

     // Any other public methods

protected:
     Singleton()
     {
          // Constructor code goes here.
     }
     ~Singleton()
     {
          // Destructor code goes here.
     }

     // And any other protected methods.
}

[edit] Non-Thread-Safe

This version doesn't require any operating-system or platform-specific features, but it is not safe in a multi-threaded environment.

class Singleton
{
public:
     static Singleton* Instance()
     {
          // Since it's a static variable, if the class has already been created,
          // It won't be created again.
          static Singleton myInstance;

          // Return a pointer to our mutex instance.
          return &myInstance;
     }

     // Any other public methods

protected:
     Singleton()
     {
          // Constructor code goes here.
     }
     ~Singleton()
     {
          // Destructor code goes here.
     }

     // And any other protected methods.
}

[edit] D

module singleton ;
import std.stdio ;
import std.thread ;
import std.random ;
import std.c.time ;
 
class Dealer {
  private static Dealer me ;
  static Dealer Instance() {
    writefln("   Calling Dealer... ") ;
    if(me is null) // Double Checked Lock
      synchronized  // this part of code can only be executed by one thread a time
        if(me is null) 
          me = new Dealer ;     
    return me ;
  }
  private static string[] str = ["(1)Enjoy", "(2)Rosetta", "(3)Code"] ;
  private int state ;
  private this() { 
    for(int i = 0 ; i < 3 ; i++) { 
      writefln("...calling Dealer... ") ;
      msleep(rand() & 2047) ;
    }
    writefln(">>Dealer is called to come in!") ;
    state = str.length - 1 ;
  }
  Dealer nextState() {
    synchronized(this) // accessed to Object _this_ is locked ... is it necessary ???
      state = (state + 1) % str.length ;
    return this ;
  }
  string toString() { return str[state] ; }   
}
 
class Coder : Thread {
  private string name_ ;
  Coder hasName(string name) {  name_ = name ; return this ; }
  override int run() {
    msleep(rand() & 1023) ;
    writefln(">>%s come in.", name_) ;
    Dealer single = Dealer.Instance ;
    msleep(rand() & 1023) ;
    for(int i = 0 ; i < 3 ; i++) {
      writefln("%9s got %-s", name_, single.nextState) ;
      msleep(rand() & 1023) ;
    }
    return 0 ;
  }
}
 
void main() { 
  Coder x = new Coder ; 
  Coder y = new Coder ; 
  Coder z = new Coder ; 
 
  x.hasName("Peter").start() ;
  y.hasName("Paul").start() ;
  z.hasName("Mary").start() ; 
 
  x.wait ;  y.wait ;  z.wait ;  
}

Sample Output:

>>Mary come in.
   Calling Dealer...
...calling Dealer...
>>Peter come in.
   Calling Dealer...
>>Paul come in.
   Calling Dealer...
...calling Dealer...
...calling Dealer...
>>Dealer is called to come in!
     Mary got (1)Enjoy
    Peter got (2)Rosetta
     Mary got (3)Code
     Paul got (1)Enjoy
    Peter got (2)Rosetta
     Paul got (3)Code
     Paul got (1)Enjoy
     Mary got (2)Rosetta
    Peter got (3)Code

[edit] E

Since E uses closure-style objects rather than classes, a singleton is simply an object which is defined at the top level of the program, not inside any method. There are no thread-safety issues since the singleton, like every other object, belongs to some particular vat (but can be remotely invoked from other vats).

def aSingleton {
  # ...
}

[edit] Io

Io does not have globals. But it is easy to make singleton objects:

 Singleton := Object clone
 Singleton clone = Singleton

[edit] Objective-C

[edit] Non-Thread-Safe

(Using Cocoa's NSObject as a base class)

// SomeSingleton.h
@interface SomeSingleton : NSObject
{
  // any instance variables
}

+ (SomeSingleton*)sharedInstance;

@end

// SomeSingleton.m
@implementation SomeSingleton

+ (SomeSingleton*) sharedInstance
{
   static sharedInstance = nil;
   if(!sharedInstance) {
      sharedInstance = [[SomeSingleton alloc] init];
   }
   return sharedInstance;
}

@end

[edit] Java

[edit] Thread-safe

Double-checked locking; only use with Java 1.5+

 
class Singleton
{
    private static Singleton myInstance;
    public static Singleton getInstance()
    {
        if (myInstance == null)
            synchronized(Singleton.class)
                if (myInstance == null)
                    myInstance = new Singleton();
 
        return myInstance;
    }
 
    protected Singleton()
    {
        // Constructor code goes here.
    }
 
    // Any other methods
}
 

[edit] Non-Thread-Safe

 
class Singleton
{
    private static Singleton myInstance;
    public static Singleton getInstance()
    {
        if (myInstance == null)
            myInstance = new Singleton();
 
        return myInstance;
    }
 
    protected Singleton()
    {
        // Constructor code goes here.
    }
 
    // Any other methods
}