Addison Wesley: Modern C++ Design: Generic Programming and Design Patterns Applied by By Andrei Alexandrescu

Author:By Andrei Alexandrescu
Language: eng
Format: mobi
ISBN: 0201704315
Publisher: Addison Wesley
Published: 2011-10-03T15:23:26+00:00

Singleton& Singleton::Instance()

{

if (!pInstance_)

{

Lock guard(mutex_);

pInstance_ = new Singleton;

}

return *pInstance_;

}

Now the overhead is gone, but the race condition is back. The first thread passes the if test, but just when it is about to enter the synchronized portion of code, the OS scheduler interrupts it and passes control to the other thread. The second thread passes the if test itself (and to no one's surprise, finds a null pointer), enters the synchronized portion, and completes it. When the first thread is reactivated, it also enters the synchronized portion, but it's just too late—again, two Singleton objects are constructed.

This seems like one of those brainteasers with no solution, but in fact there is a very simple and elegant one. It's called the Double-Checked Locking pattern.

The idea is simple: Check the condition, enter the synchronized code, and then check the condition again. By this time, rien ne va plus—the pointer is either initialized or null all right. The code that will help you to understand and savor the Double-Checked pattern follows. Indeed, there is beauty in computer engineering.

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