Re: Singleton_pattern and Thread Safety
On 10/12/2010 15:03, Fred Zwarts wrote:
"Leigh Johnston"<leigh@i42.co.uk> wrote in message
news:ItOdncr4ttt9qp_QnZ2dnUVZ8gSdnZ2d@giganews.com
On 10/12/2010 13:59, Fred Zwarts wrote:
"Leigh Johnston"<leigh@i42.co.uk> wrote in message
news:rsudnaHfQqi7tZ_QnZ2dnUVZ8q-dnZ2d@giganews.com
On 10/12/2010 09:52, James Kanze wrote:
On Dec 9, 5:05 pm, Marcel M?ller<news.5.ma...@spamgourmet.com>
wrote:
Pallav singh wrote:
i have a query using given singleton that its not thread Safe ?
Since function getInstance() is returning the static object
singleton class AS far my knowlege, static object is
intialized only first time when control reaches there. The
second time control Thread reached there , compiler skipps
the initialization part.
That's right.
// Source file (.cpp)
Singleton& Singleton::getInstance()
{
// Static Variables are initialized only first time Thread of
// Execution reaches here first time.
static Singleton instance;
This line is not guaranteed to be thread safe. In some
implementation it is safe.
In practice, it will be thread safe *if* the first call to
getInstance occurs before threading starts. If threading
doesn't start before entering main (normally an acceptable
restriction), then just declaring a variable with static
lifetime which is initialized by getInstance() is sufficient,
e.g. (at namespace scope):
Singleton* dummyForInitialization =&Singleton::getInstance();
Note that the above still risks order of destruction issues;
it's more common to not destruct the singleton ever, with
something like:
namespace {
Singleton* ourInstance =&Singleton::instance();
Singleton&
Singleton::instance()
{
if (ourInstance == NULL)
ourInstance = new Singleton;
return *ourInstance;
}
}
(This solves both problems at once: initializing the variable
with a call to Singleton::instance and ensuring that the
singleton is never destructed.)
James "Cowboy" Kanze's OO designs includes objects that are never
destructed but leak instead? Interesting. What utter laziness
typical of somebody who probably overuses (abuses) the singleton
pattern. Singleton can be considered harmful (use rarely not
routinely).
As far as I can see it does not leak.
Up to the very end of the program the ourInstance pointer keeps
pointing to the object and can be used to access the object.
This is a well known technique to overcome the order of destruction
issue.
Of course it is a memory leak the only upside being it is a singular
leak that would be cleaned up by the OS as part of program termination
rather than being an ongoing leak that continues to consume memory.
So, it is a matter of definition whether you want to call that a leak.
Usually something is acalled a leak if an object is no longer accessible,
because the pointer to the object went out of scope, or was assigned
a diferent value.
It is lazy. As far as it being a "well known technique" I have
encountered it before when working on a large project with many team
members but that does not justify its use; it was a consequence of
parallel development of many sub-modules with insufficient time set
aside for proper interop design and too much risk associated with
"fixing" it.
It is not necessarily lazy. The order of destruction of global objects
is not always predictable. Why spending time for a complex
solution, if it serves no purpose and makes the code much more
difficult to read?
What do you mean by global objects? If you mean objects defined at
namespace scope or static class member objects then you should avoid
having such objects in more than one translation unit modulo the advice
that one should avoid globals as they are definitely considered harmful.
Singletons are nothing more than disguised global variables.
Destruction is the opposite of construction; destruction (proper
cleanup) is not an intractable problem. /delete/ what you /new/.
Why? If destruction does not serve any purpose?
Is it a fixed rule so that you don't need to think about it?
If you can define construction then you can also define destruction;
why? Code re-use is one reason; e.g. a class which was initially a
singleton may suddenly be required to be instantiated more than once. A
class shouldn't really care about how many instances of it will be
created; ideally all objects should be destroyed on program termination
the only exceptions to this being abnormal program termination (e.g.
unhandled exception) or a program that is supposed to never terminate.
/Leigh