Class to get a thread safe count of previous Instances

There have been many examples,posted on CodeGuru, of how to check for a previously running instance of an application. Some used DDE, others mutexs but what struck me as the easiest suggestion was to use a 'shared section' to store a flag - accessable by every instance.

The following class uses that technique, along with a couple of the 'interlocked' functions to make it threadsafe. Instead of a simple flag, a count of running instances is kept. The count obtained by the class is only a snapshot.

To use the class simply create a static instance of it. Then check the 'Count()' method to determine how many instances of your application where running before the CPreviousInstance object was instantiated.

Code tested on WinNT 4.0 spk 4 VC6.0 spk 2


// CPreviousInstance.h

class CPreviousInstance  
{
public:
	CPreviousInstance();
	virtual ~CPreviousInstance();

	LONG Count() const
	{
		return m_previous;
	}

private:
	static LONG		s_count;
	LONG			m_previous;

	CPreviousInstance(const CPreviousInstance&);
	CPreviousInstance& operator=(const CPreviousInstance&);
};

// CPreviousInstance.cpp

// static instance count stored in a shared read/write section

#pragma data_seg("Instance")

LONG  CPreviousInstance::s_count = 0;

#pragma data_seg()
#pragma comment(linker,"/section:Instance,rws")


// Construction/Destruction

CPreviousInstance::CPreviousInstance():
m_previous(0)
{
	m_previous = ::InterlockedIncrement(&s_count);
	--m_previous;
}

CPreviousInstance::~CPreviousInstance()
{
	::InterlockedDecrement(&s_count);
}