On Thursday, January 05, 2012 22:15:32 asm wrote:
> how can i implementing the singleton pattern in D?
The same way that you would in C++ or Java, except that unlike you have static
constructors that you can use to initialize them, and unlike both, if it's not
shared, then it's a singleton per thread and no locking is required.
If you're dealing with a multi-threaded singleton, technically, it should be
possible to use double-checked locking with shared, but apparently bugs with
shared make that unsafe at present. So, if you didn't initialize it in a
shared static constructor, either you lock every time that you fetch the
singleton or you use an extra bool for indicating that it's been initialized.
Single threaded:
T singleton()
{
static T instance;
if(!instance)
instance = new T();
return instance;
}
Multi-threaded:
shared(T) singleton()
{
static shared T instance;
static bool initialized = false;
if(!initialized)
{
synchronized
{
if(!instance)
instance = new shared(T)();
}
initialized = true;
}
return instance;
}
Once the bugs with shared with regards to fences have been fixed though, the
shared version can look more like
shared(T) singleton()
{
static shared T instance;
if(!instance)
{
synchronized
{
if(!instance)
instance = new shared(T)();
}
}
return instance;
}
Regardless, if you use a shared static constructor, you can avoid the locking
(it's just not lazily initialized then), and of course, the singly threaded
one can use a static constructor if you don't care about lazy initialization.
If you use a static constructor though, the instance would not be declared
inside the singleton function (and it doesn't have to be here either - it's
just shorter to do it this way in an example).
- Jonathan M Davis
