• yaneurao (25/25) Jan 17 2004 in C++ , thread safe singleton class is almost written by double checked
• Matthew (7/32) Jan 17 2004 I doubt it.
• Sean L. Palmer (30/40) Jan 20 2004 This might be related to the current discussion about auto object creati...

yaneurao <yaneurao_member pathlink.com> writes:

in C++ , thread safe singleton class is almost written by double checked
locking style.
(more imfomation about this, see 'pattern hatching' wriiten by John Vlissides)

in D , at first I wrote following program.

----------------------------------------------------------------
template singleton(T) { class singleton {
public:
static T get() {
if (!t) {
synchronized {
if (!t) t = new T;
}
}
return t;
}
private:
static T t;
}}
----------------------------------------------------------------
but I can't guess whether it works well or not.
double-checked locking is the idiom that should never be used in Java.

for more imfomation about this:
http://www-106.ibm.com/developerworks/java/library/j-dcl.html?dwzone=java

I want to know whether double checked locking is valid or not in D.

yaneurao.

"Matthew" <matthew.hat stlsoft.dot.org> writes:

I doubt it.

That's one of the reasons why making singletons is not a simple thing.
(Since D has modules, I tend to agree with Sean that they're not really
needed anyway.)

"yaneurao" <yaneurao_member pathlink.com> wrote in message
news:bud0lr$1ekq$1 digitaldaemon.com...
 in C++ , thread safe singleton class is almost written by double checked
 locking style.
 (more imfomation about this, see 'pattern hatching' wriiten by John

Vlissides)
 in D , at first I wrote following program.

 ----------------------------------------------------------------
 template singleton(T) { class singleton {
 public:
 static T get() {
 if (!t) {
 synchronized {
 if (!t) t = new T;
 }
 }
 return t;
 }
 private:
 static T t;
 }}
 ----------------------------------------------------------------
 but I can't guess whether it works well or not.
 double-checked locking is the idiom that should never be used in Java.

 for more imfomation about this:
 http://www-106.ibm.com/developerworks/java/library/j-dcl.html?dwzone=java

 I want to know whether double checked locking is valid or not in D.

 yaneurao.

"Sean L. Palmer" <palmer.sean verizon.net> writes:

This might be related to the current discussion about auto object creation
syntax.

Although I suppose these guys have one point... one of the purposes of
singletons is to allow third parties to inject code into the system by
deriving from the singleton base... for that reason, point of instantiation
should be left out of the base so that it may be created by the client
(unless you can get the base class to know enough about the derived class,
e.g. the strangely recursive inheritance pattern).

There isn't a way to accomplish that (easily) with the module system aside
from public import.  But once you have that, you start having a kind of one
instance, semi-extendable kind of class/object combination, which almost
exactly fits the definition of singleton.

Every C++ singleton implementation I've seen has had problems though... it's
as if the language somehow goes out of its way to make that kind of thing
difficult.  Personally I blame the loose linking / inter-module
initialization order standards.  Also compilers tend to strip out isolated
global objects that are never used by anything, so you cannot for instance
put your program entry point inside a global object ctor.  If it weren't for
that, global object ctors/dtors would be a good way to write module
init/shutdown code.

It seems to me that if the module system is in fact intended to supercede
the concept of singleton, that it should provide the capacity for public
import and it should do this kind of thread-safety automatically.

Or maybe use of the synchronized keyword, as in:

synchronized module mysingleton;

Or maybe

synchronized static this() { ... }

Sean

"Matthew" <matthew.hat stlsoft.dot.org> wrote in message
news:bud3a3$1ik1$1 digitaldaemon.com...
 I doubt it.

 That's one of the reasons why making singletons is not a simple thing.
 (Since D has modules, I tend to agree with Sean that they're not really
 needed anyway.)

 "yaneurao" <yaneurao_member pathlink.com> wrote in message
 news:bud0lr$1ekq$1 digitaldaemon.com...
 in C++ , thread safe singleton class is almost written by double checked
 locking style.
 (more imfomation about this, see 'pattern hatching' wriiten by John

 Vlissides)