On Tuesday, 9 April 2019 at 10:53:49 UTC, Ron Tarrant wrote:
On Monday, 8 April 2019 at 14:56:46 UTC, Mike Parker wrote:
In the subsequent sections, I show both long and short
(eponymous) forms of enum and function templates.
In your book, Mike, you stated:
Remember, a template is only instantiated once for each set of
arguments and
the same instantiation can be repeated in multiple modules
throughout a
program. If each instantiation were scoped locally, the
template would no
longer work as expected.
That leads me to speculate that it should be possible to use a
class template as a sort of singleton. But, because a class
template (from what I understand currently) needs to be
instantiated using 'new,' I'm thinking maybe it isn't possible.
Can you (or someone else) clear this up for me, please?
Instantiating a class template is not like instantiating a single
instance of an object. What it does is create an implementation
of the class, not an instance. So given this:
class Foo(T) {
T item;
}
Then this instantiation (which, byt the way, does not need new --
an alias will do the trick):
alias IntFoo = Foo!int;
Essentially does this:
class FooWithInt {
int item;
}
Of course, the symbol will be different than 'FooWithInt', but
the point is for every T there has to be a uniquely named
implementation of Foo.
What I have in mind is a template wrapped around a GTK
AccelGroup that could then be instantiated in the MainWindow as
well as inside any MenuItem needing a keyboard shortcut
assignment.
Off the top of my head, to get a Singleton template, you could
implement all of your singleton plumbing (thread safety if you
need it, etc) in the template and add a `static _instance` member
just as you would for any non-templated singleton:
class Singleton(T) {
private static Singleton!T _instance;
static Singleton!T instance() {
if(_instance is null) {
_instance = new Singleton!T;
}
...
}
private T _thing;
...
}
And you can then instantiate directly or, more conveniently, use
an alias:
alias Accelerators = Singleton!AccelGroup;
Then:
Accelerators.instance.doSomething();
