Re: Double-checked locking: is this alternative OK?

From:
"david.baird" <dhbaird@gmail.com>
Newsgroups:
comp.lang.c++
Date:
20 Mar 2007 22:53:46 -0700
Message-ID:
<1174456426.492776.141460@y66g2000hsf.googlegroups.com>
On Mar 17, 8:31 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:

    if (instance == NULL) {
        LockThing();
        instance = new SomeObject();
        UnlockThing();
    }
    return instance;


I think this can be improved to:

    if (instance == NULL) {
        LockThing();
        if (instance == NULL) {
            instance = new SomeObject();
        }
        UnlockThing();
    }
    return instance;

This is mentioned in [1] as "The Double-Checked Locking Pattern."

   Thread 1 Thread 2
 LockThing() - succeeds
                         LockThing() - fails, waits
 new SomeObject() . // waiting
 UnlockThing() . // waiting
 return instance
                         new SomeObject() // overrides instance


The above code fixes this, instead of calling "new SomeObject()":
   Thread 1 Thread 2
                         Checks if instance is NULL
                         (Since instance is not NULL,
                          do *not* call "new SomeObject()")

                         UnlockThing()
                         return instance // different instance


[1] "Modern C++ Design" Andrei Alexandrescu

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