Singleton
You are encouraged to solve this task according to the task description, using any language you may know.
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.
Ada
Non Thread Safe
<ada>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;</ada>
<ada>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;</ada>
Thread Safe
<ada>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;</ada>
<ada>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;</ada>
C++
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. }
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. }
D
<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 ;
}</d> 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
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 { # ... }
Objective-C
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
Java
Thread-safe
Double-checked locking; only use with Java 1.5+ <java> 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
} </java>
Non-Thread-Safe
<java> 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
} </java>