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
type Singleton is limited private; procedure Set_Data(Item : out Singleton; Value : Integer); function Get_Data(Item : Singleton) return Integer; function Create return Singleton;
private
protected type Instance_Type is
procedure Set(Value : Integer);
function Get return Integer;
private
Data : Integer := 0;
end Instance_Type;
type Singleton is access all Instance_Type;
Instance : aliased Instance_Type;
end Protected_Singleton;</ada>
<ada>package body Protected_Singleton is
-------------- -- Set_Data -- --------------
procedure Set_Data (Item : out Singleton; Value : Integer) is
begin
if Item = null then
Item := Create;
end if;
Instance.Set(Value);
end Set_Data;
-------------- -- Get_Data -- --------------
function Get_Data (Item : Singleton) return Integer is
begin
return Instance.Get;
end Get_Data;
------------ -- Create -- ------------
function Create return Singleton is
begin
return Instance'access;
end Create;
------------------- -- Instance_Type -- -------------------
protected body Instance_Type 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_Type;
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
<java> class Singleton {
private static Singleton myInstance;
public static synchronized Singleton getInstance()
{
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>