Singleton

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.

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 );

          // 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.
}

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