Re: Thread safe Singleton class

Bart van Ingen Schenau <>
Fri, 26 Jun 2009 16:15:09 +0200
Udit Sharma 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

class Singleton
static Singleton instance;
Singleton(const Singleton&){}
Singleton& operator=(Singleton const&){}
static Singleton& getInstance();

#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 is not directly related to thread safety, but your code has the
potential of running into C++'s static initialisation fiasco: If you
have objects with static lifetime and namespace scope (or as static
members of a class), and these objects are defined in different source
files, then it is unspecified in which order these objects are
This becomes a problem if these objects can refer to each other.

For example, if you have another source file with this code:

#include "Singleton.h"
struct SingletonUser {
  SingletonUser() { Singleton::getInstance().doSomething(); }

static SingletonUser su;

Then there is a significant chance that Singleton::getInstance() gets
called BEFORE Singleton::instance was initialised.

This problem can usually be prevented by writing getInstance like this:
  Singleton& Singleton::getInstance()
    static Singleton instance;
    return instance;

But that has potential threading issues, if the first call can occur
from multiple threads and the compiler does not add sufficient


Bart v Ingen Schenau
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