Re: Confused about a thread-safe singleton example.
On Dec 5, 9:07 am, James Kanze <james.ka...@gmail.com> wrote:
On Dec 4, 4:58 pm, Maxim Yegorushkin <maxim.yegorush...@gmail.com>
wrote:
[]
So, a global variable can be an instance of a singleton class,
can it not?
It would depend on how the singleton is implemented. The
oldest, traditional implementation (use a static counter,
incremented in the constructor, and assert that it is never
greater than 1 after the incrementation) certainly allows it.
Most of the C++ implementations wouldn't allow it, requiring you
to go through a special function. More generally, I think, this
is usually the case in other languages as well, since by making
the constructor private, and requiring the function to be
called, makes it impossible to write compilable code which would
instantiate more than one instance.
It's not a perfect solution, however---as I think you
mentionned, it doesn't allow derivation, at least not easily.
It was mentioned by Alan Johnson.
Personally, I prefer deriving from interface classes only, for
anything else composition seems to me to be a superior approach, so
whether something can be derived from is the last thing I would care.
And all those calls to instance() ARE a real pain in the neck.
The problem, IMO, is that it is supposed to limit the number of
instances on the type level. Yet, there is a price to be paid at run-
time for every usage of ::instance() call (albeit small). On the other
hand, accessing good old global variables (or global references to
abstract interfaces) has zero price and seem to work quite well when I
need one instance of something and it does not require special syntax
for accessing that instance (less typing at the least).
I have my one instance and I just can't be bothered to enforce that by
making the class a singleton, for the very same reason that I can't be
bothered to prohibit derivation from classes that are not supposed to
be derived. If someone wants to shoot his foot off I won't mess with
the person's good will, experience work best when it is obtained first
hand ;).
--
Max