Singleton Method

[getviswa]

Hi All,

Please suggest me the link or sample code for the singleton method.

1) Need to create single instance and need to use that instance in entire project.
2) Should not allow to create new instance.

#include <stdio.h>
#include <iostream>
using namespace std;
class Sample
{
	private:
	int test;
	public:
	int setValue(int a)
	{
		test=a;
	}

	int getValue()
	{
		cout<<"test value = "<<test;
	}
};
class CMySingleton
{


	private:
  	CMySingleton() {};   
	static Sample *singleton;

	public:
  	static Sample* Instance()
  	{
		singleton=new Sample();
	    	return singleton;
  	}



};
int main()
{
	Sample *obj=CMySingleton::Instance();
	cout<<"\n "<<&obj;
	Sample *obj1=CMySingleton::Instance();
	cout<<"\n "<<&obj1;
	return 0;

}

It throws error

/tmp/ccFuSDhv.o: In function `CMySingleton::Instance()':
singleton.cpp.text._ZN12CMySingleton8InstanceEv[CMySingleton::Instance()]+0x17): undefined reference to `CMySingleton::singleton'
singleton.cpp.text._ZN12CMySingleton8InstanceEv[CMySingleton::Instance()]+0x1e): undefined reference to `CMySingleton::singleton'

Thanks
-Viswa

[ahoodin]

To Fix your sample, you will need to add the line in bold.

	static Sample* Instance()
  	{
		singleton=new Sample();
	    	return singleton;
  	}



};
Sample *CMySingleton::singleton = NULL;

There are faqs in CG that cover singleton.

HTH,

[ninja9578]

That's not a good idea. Singletons should clean themselves up, if you create your singleton on the heap, then you are supposed to delete it.

static Sample & Instance(){
     Sample singleton;
     return singleton;
}

Here it gets initialized on first call, and gets automatically destroyed properly when the program exits. It also eliminates the need for a global.

[ahoodin]

Give him a break.

They need to learn, and the way he is doing it is a simple singleton pattern.

http://forums.codeguru.com/showthread.php?t=344782

[monarch_dodra]

That's not a good idea. Singletons should clean themselves up, if you create your singleton on the heap, then you are supposed to delete it.

static Sample & Instance(){
     static Sample singleton;
     return singleton;
}

Here it gets initialized on first call, and gets automatically destroyed properly when the program exits. It also eliminates the need for a global.

Err... with an extra static in there... right?

[monarch_dodra]

That's the basic idea, but remember you want to create only a single instance, so you check if singleton is null first:

	public:
  	static Sample* Instance()
  	{
		if(!singleton)
			{singleton=new Sample();}
	    	return singleton;
  	}

There are as many ways to write a singleton as fingers on your hand. Don't worry about it too much, as long as you have something working.

[ahoodin]

Yes that is correct MD. Nice catch!

[STLDude]

That's the basic idea, but remember you want to create only a single instance, so you check if singleton is null first:

	public:
  	static Sample* Instance()
  	{
		if(!singleton)
			{singleton=new Sample();}
	    	return singleton;
  	}

This will fail in multithreaded environment.

[monarch_dodra]

This will fail in multithreaded environment.

Are you singling out my implementation specifically? Because the entire pattern is not thread safe, so its not like there's an alternative.

[ninja9578]

Oops, I missed a static.

In multithreaded environments, it is most common (that I've seen) to call all of the singleton methods in main() before any threads get launched.

[S_M_A]

The static one is thread safe in C++11 if I don't remember wrong.

[STLDude]

The static one is thread safe in C++11 if I don't remember wrong.

You are right:

§ 6.7.4
.. If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization.

[ninja9578]

Actually, now that I think about it. LLVM has an option for thread-safe statics too, even without C++11.