Re: Thread safe Singleton class

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 26 Jun 2009 12:21:17 -0700 (PDT)
Message-ID:
<b9a1f5fb-3afe-4109-9a93-8af72e16e891@v2g2000vbb.googlegroups.com>
On Jun 26, 12:50 pm, Udit Sharma <sharmaa.u...@gmail.com> wrote:

I was informed by my colleague that the following code is not
thread safe but I still don't know why because his arguments
weren't very convincing.

//Singleton.h
class Singleton
{
private:
static Singleton instance;
Singleton(){}
Singleton(const Singleton&){}
Singleton& operator=(Singleton const&){}
public:
static Singleton& getInstance();
};

//Singleton.cpp
#include "Singleton.h"
Singleton Singleton::instance;

Singleton& Singleton::getInstance()
{
return instance;
}

Since am maintaining a static instance this should be created
only once irrespective of how many threads are accessing the
getInstance() function at the same time. So is this code not
thread safe?


It depends. Principly, it's not safe from order of
initialization problems, which means that using the singleton in
the constructors of static objects is problematic. If one of
these constructors starts additional threads, there may be a
threading problem as well.

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