Re: What is the problem with writing singleton in multithreaded enviroment

From:

"James Kanze" <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

17 Jan 2007 11:23:32 -0500

Message-ID:

<1169022210.766884.279600@l53g2000cwa.googlegroups.com>

Joshua Lehrer wrote:

James Kanze wrote:

Ronen Yacov wrote:

When declaring a singleton, we write:

static CMySingle::instance()
{
static CMySingle instance;
return instance;
}

The simple answer is because the C++ standard doesn't define the
semantics of this construct in a multithreaded environment. A
more complex answer is that the standards which do define
multithreaded semantics (e.g. Posix) only define it for C, and
this construct has no equivalent in C. And the practical
answer, of course, is that unless the compiler does something
special (and as far as I know, only g++ does), there's a very

Dec/Compaq/HP C++ on VMS / Itanium automatically guards the
construction of function scoped statics guaranteeing the "proper"
multithreaded behavior with little to no overhead added to
non-multithreaded uses.

That's good to know, as it represents significant existing
practice. If I understand the latest papers (I've just scanned
them), the current tendency in the standards committee is to add
a new keyword to tell the compiler that it must make them safe;
if several compilers already make them safe systematically,
without excessive overhead, then there's really no need for a
new keyword.

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