Re: Singleton_pattern and Thread Safety

Joshua Maurice <>
Fri, 10 Dec 2010 19:42:13 -0800 (PST)
On Dec 10, 7:40 pm, Joshua Maurice <> wrote:

On Dec 10, 7:17 pm, Leigh Johnston <> wrote:

On 11/12/2010 03:12, Joshua Maurice wrote:

On Dec 10, 6:38 pm, Leigh Johnston<> wrote:

On 11/12/2010 02:23, Leigh Johnston wrote:

On 10/12/2010 23:31, Ian Collins wrote:

On 12/11/10 10:08 AM, Leigh Johnston wrote:

On 10/12/2010 20:39, Ian Collins wrote:

On 12/11/10 09:21 AM, Leigh Johnston wrote:

Not considering object destruction when designing *new* classes=

 is bad

practice IMO. Obviously there may be problems when working with
pre-existing designs which were created with a lack of such

A programmer seldom has the benefit of a green field design. Eve=

n when

he or she does, there are still the dark and scary corners of th=


language where undefined behaviour lurks. Order of destruction
issues is
one such corner, especially when static objects exist in multipl=


compilation units.

I am well aware of the unspecified construction/destruction order
associated with globals in multiple TUs and that is primary reaso=

n why

this method of James's should be avoided. The order of destructio=

n of

"Meyers Singleton" objects *is* well defined for example although=


the "Meyers Singleton" method thread safe is not completely trivi=


That is another pattern I use, but as you say, it has issues of it=

s own.

Normally I instantiate all my singletons up front (before threading=

) but

I decided to quickly roll a new singleton template class just for t=


fun of it (thread-safe Meyers Singleton):

namespace lib
template<typename T>
class singleton
static T& instance()
if (sInstancePtr != 0)
return static_cast<T&>(*sInstancePtr);
{ // locked scope
lib::lock lock1(sLock);
static T sInstance;
{ // locked scope
lib::lock lock2(sLock); // second lock should emit memory barrier h=


sInstancePtr =&sInstance;
return static_cast<T&>(*sInstancePtr);
static lib::lockable sLock;
static singleton* sInstancePtr;

template<typename T>
lib::lockable singleton<T>::sLock;
template<typename T>
singleton<T>* singleton<T>::sInstancePtr;

Even though a memory barrier is emitted for a specific implementatio=

n of

my lockable class it obviously still relies on the C++ compiler not
re-ordering stores across a library I/O call (acquiring the lock) bu=

t it

works fine for me at least (VC++). I could mention volatile but I
better not as that would start a long argument. Roll on C++0x.

If I'm reading your code right, on the fast path, you don't have a
barrier, a lock, or any other kind of synchronization, right? If yes,
you realize you've coded the naive implementation of double checked?
You realize that it's broken, right? Have you even read
To be clear, this has undefined behavior according to the C++0x
standard as well.

I am aware of double checked locking pattern yes and this is not the
double checked locking pattern (there is only one check of the pointer
if you look). If a pointer read/write is atomic is should be fine (o=


the implementation I use it is at least).

You've hidden the second check with the static keyword.

Example: Consider:

  SomeType& foo()
    static SomeType foo;
    return foo;

For a C++03 implementation, it's likely implemented with something

  SomeType& foo()
    static bool b = false; /*done before any runtime execution, sto=


in the executable image */
    static char alignedStorage[sizeof(SomeType)]; /*with some magic
for alignment */
    if ( ! b)
      new (alignedStorage) SomeType();
    return * reinterpret_cast<SomeType*>(alignedStorage);

That's your double check.

For C++0x, it will not be implemented like that. Instead, it will be
implemented in a thread-safe way that makes your example entirely

Err, that should be:

  SomeType& foo()
    static bool b = false; /*done before any runtime execution, stored
in the executable image */
    static char alignedStorage[sizeof(SomeType)]; /*with some magic
for alignment */
    if ( ! b)
      new (alignedStorage) SomeType();
      b = true;
    return * reinterpret_cast<SomeType*>(alignedStorage);

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