Re: Threadsafe singletons

From:

"Matthias Hofmann" <hofmann@anvil-soft.com>

Newsgroups:

comp.lang.c++.moderated,comp.programming.threads

Date:

1 Aug 2006 17:34:39 -0400

Message-ID:

<44cfb4c1$0$18509$9b4e6d93@newsread2.arcor-online.net>

"kanze" <kanze@gabi-soft.fr> schrieb im Newsbeitrag
news:1154436138.221933.71380@p79g2000cwp.googlegroups.com...

What exactly is the problem with creation of a local static
object and threads?

The code generated by most compilers isn't thread safe.
Typically, there will be a hidden static bool which says whether
the object has been initialized or not; the compiler tests this,
and if it is false, calls the constructor, and then sets it to
true. If no particular steps are taken by the compiler, this
isn't thread safe.

Is this hidden static bool shared by the threads? I can imagine that there
is a problem if the bool is shared while the object is not or vice versa.
Otherwise I don't understand what could go wrong.

[...]

Formally, there is no guarantee that static variables are
constructed before entering main, so you have no guaranteed that
your s_init object (or my ourInstance pointer) will be
initialized before entering main.

Really! Are you referring to the C++ standard?

It is described in 3.6.2/3. If I understand this part of the
standard correctly, it does, however, guarantee that the
ourInstance pointer will be initialized before it is first
used.

It guarantees that the pointer will be initialized before the
first use of anything defined in the translation unit. In
particular, it guarantees that the pointer will be initialized
before the first call to Singleton::GetInstance().

And what if nothing is used except for the pointer itself? Strangely, I
found no text in the standard that explicitly guarantees that an object will
be initialized before it is first used... Or maybe I just overlooked it?

--
Matthias Hofmann
Anvil-Soft, CEO
http://www.anvil-soft.com - The Creators of Toilet Tycoon
http://www.anvil-soft.de - Die Macher des Klomanagers

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