Re: Singleton_pattern and Thread Safety

From:
Leigh Johnston <leigh@i42.co.uk>
Newsgroups:
comp.lang.c++
Date:
Tue, 14 Dec 2010 14:45:49 +0000
Message-ID:
<FIudnQcZOI-IHprQnZ2dnUVZ8j6dnZ2d@giganews.com>
On 14/12/2010 10:05, James Kanze wrote:

On Dec 13, 6:47 pm, Leigh Johnston<le...@i42.co.uk> wrote:

On 13/12/2010 18:40, James Kanze wrote:

On Dec 13, 6:17 pm, Leigh Johnston<le...@i42.co.uk> wrote:

On 13/12/2010 11:45, James Kanze wrote:


      [...]

As you are doing it wrong it is neither defensive programming
nor sound engineering.


Again, I'd suggest you read my code very, very carefully. (I'll
admit that it's not immediately obvious as to why it works. But
it's been reviewed several times by leading experts, and never
found wanting.)


You are doing it wrong. I say again if you have more than one of your
leaking singletons defined in more than one TU the construction order of
your leaking singletons is unspecified.

This is your code:

      namespace {

      Singleton* ourInstance =&Singleton::instance();

      Singleton&
      Singleton::instance()
      {
          if (ourInstance == NULL)
              ourInstance = new Singleton;
          return *ourInstance;
      }
      }

The ourInstance *pointer* is a global object (albeit with internal
linkage) which you are initializing with a dynamic allocation wrapped in
a function.


The ourInstance pointer is *not* a global object. No code
outside the above can access it. (There is a simple error in
the posted code: the function Singleton::instance() shouldn't be
in unnamed namespace, since the class Singleton obviously isn't
in unnamed namespace.)

If you have more than such initialization in more than one
TU the order of the initializations is unspecified.


Yes, but it doesn't matter:

  -- ourInstance is initialized to null before any C++ code is
     executed (zero initialization).

  -- client code cannot access ourInstance; all accesses go
     through Singleton::instance().

  -- Singleton::instance() checks for null, and initializes the
     pointer if necessary, regardless of when it is called. This
     is the classic implementation of the singleton pattern, and
     ensures that there is no order of initialization problem
     when Singleton is used.

  -- The "formal" initialization of ourInstance (after the = sign
     in its definition ensures that Singleton::instance() is
     called at least once during static initialization, and thus
     that the pointer is initialized before entering main (in
     practice, at least), and thus normally before threading
     starts. This is only necessary for thread safety---once the
     pointer has been correctly initialized, the code is thread
     safe without any synchronization.


Of course it matters. If two of your singletons defined in different
TUs do not reference each other but do reference a third object *during
construction* the order of these references is unspecified.

/Leigh

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