Re: Singleton_pattern and Thread Safety
On Dec 10, 6:38 pm, Leigh Johnston <le...@i42.co.uk> 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
consideration.
A programmer seldom has the benefit of a green field design. Even wh=
en
he or she does, there are still the dark and scary corners of the
language where undefined behaviour lurks. Order of destruction
issues is
one such corner, especially when static objects exist in multiple
compilation units.
I am well aware of the unspecified construction/destruction order
associated with globals in multiple TUs and that is primary reason wh=
y
this method of James's should be avoided. The order of destruction of
"Meyers Singleton" objects *is* well defined for example although mak=
ing
the "Meyers Singleton" method thread safe is not completely trivial.
That is another pattern I use, but as you say, it has issues of its ow=
n.
Normally I instantiate all my singletons up front (before threading) bu=
t
I decided to quickly roll a new singleton template class just for the
fun of it (thread-safe Meyers Singleton):
namespace lib
{
template <typename T>
class singleton
{
public:
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 here
sInstancePtr = &sInstance;
}
}
return static_cast<T&>(*sInstancePtr);
}
private:
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 implementation 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) but 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
http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf
?
To be clear, this has undefined behavior according to the C++0x
standard as well.