Re: Is this Singleton class thread-safe?

James Kanze <>
Thu, 15 Nov 2007 04:24:03 -0800 (PST)
On Nov 14, 6:35 pm, Angus <> wrote:

I have written a singleton class like this:



// Instance


template <typename T>
T * CSingleton<T>::Instance(
) throw()
    static T inst;
    return &inst;

Is this thread safe?

Maybe. You've cut the documentation which states its
requirements, so there's no way to tell.

Could two threads (with cpu context switching) possibly create
more than one instance of the class passed as a template? Is
this possible?

Possibly. It depends on whether two threads have a right to do
this or not.

If you want to allow multiple threads to call Instance without
external locking, you'll probably have to take some precautions

If it is I presume I must lock using mutex or some mechanism
before line - static T inst; and just after it?

There are other solutions, depending on the context:

 -- You could just document that the user must externally
    synchronize calls to Instance. If the user needs external
    synchronization to use the returned object anyway, this is
    perfectly acceptable, and doubtlessly the simplest solution.

 -- If you don't need to support multithreading before main is
    called, something like:

        T* Singleton<T>::ourInstance = Singleton<T>::instance() ;

            return ourInstance == NULL
                ? new T
                : ourInstance ;

    can be used. This works if (and only if) instance() is
    called at least once before threading starts; the
    initializer of ourInstance guarantees that it will be called
    at least once before main() is entered (in practice, if not
    necessarily formally).

    This is what I tend to do in generic solutions, or when T is
    mainly a read-only object. (Note that this has the
    additional advantage that the instance is never destructed,
    so you don't run into order of destructor problems either.)

 -- If the user absolutly needs external synchronization to use
    the object, you can provide it for him, by returning a smart
    pointer which reference counts, and whose last instance
    unlocks, and grab the lock before starting. Something like:

        pthread_mutex_t ourLock = PTHREAD_MUTEX_INITIALIZER ;

        struct Unlocker
            void operator()() {
                pthread_mutex_unlock( &ourLock ) ;
        } ;

        boost::shared_ptr< T >
            pthread_mutex_lock( &ourLock ) ;
            static T* theOneAndOnly = NULL ;
            if ( theOneAndOnly == NULL ) {
                theOneAndOnly = new Singleton ;
            return boost::shared_ptr< T >( theOneAndOnly,
Unlocker() ) ;

    Once again, the only lock you've acquired is one that the
    user needed anyway.

    (I don't think that this solution is appropriate for generic
    code, but it seems a nice idea for some special cases.)

James Kanze (GABI Software)
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