Thread: Singleton C++ Class - How to avoid mem leak in multi threading

Hello,
got a C++ doubt..

normally Singleton class is written like
Sing.h
==========
class Sing{
private:
Sing();
static Sing* instance;
public:
static Sing *getInstance;
};

Sing.cpp
========

Code:

#include "Sing.h"
Sing::Sing()
{
}

Sing* Sing::instance = 0;

Sing* Sing::getInstance()
{
	if (instance == 0)
	{
		instance =  new Sing();
	}
	return instance;
}

In case of singleton, dest never gets called. So, mem allocated using new does not have corresponding delete. So, this can lead to mem leak.

Hence,  i heard this can be solved using 

Sing* Sing::getInstance()
{
	static Sing instance;
	return &instance;
}

But this proc can be used only for single thread. not for multi.

Why so. Why cant it be used for multithreading??

-Regards,
molus