Re: local static is thread safe?

From:
Gerhard Menzl <clcppm-poster@this.is.invalid>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 19 Jul 2007 09:54:42 CST
Message-ID:
<f7n19u$uol$1@news.datemas.de>
ChenA wrote:

class A
{
public:
  static A & GetInstance()
  {
    static A instance;
    return instance;
  }
.........
};

I guess the GetInstance function is converted by the compiler into
something like this:

static A & GetInstance()
{
  static bool a_constructed = false;
  static uninitialized A a;
  if (!a_constructed)
  {
    a_constructed = true;
    new(&a) A; // construct it
    atexit(DestructA);
  }
  return a;
}

This is obvious not thread safe.

I don't know the c++ compiler implementation.
Is my guess right?


You are right, this is not thread-safe since the C++ standard does not
acknowledge the existence of threads - yet. Consequently, there is no
portable way to make this thread-safe. You may google for
"double-checked locking" for extensive discussions on this subject.

--
Gerhard Menzl

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