On 1/29/11 5:01 AM, Simen kjaeraas wrote:
Tomek Sowiński <j...@ask.me> wrote:
Michel Fortin napisał:
> Is there anything implementation specific in the outer struct that
provides
> ref semantics to Impl? If not, Container could be generic,
parametrized by
> Impl type.
You could provide an implementation-specific version of some functions
as an optimization. For instance there is no need to create the Impl
when asking for the length, if the pointer is null, length is zero.
Typically, const function can be implemented in the outward container
with a shortcut checking for null.
I think the reference struct can still be orthogonal to the container.
struct Ref(Impl)
{
private Impl* _impl;
ref Impl impl() @property
{
if (!impl) impl = new Impl;
return *impl;
}
static if (hasLength!Impl)
{
auto length() @property
{
return impl ? impl.length : 0;
}
}
alias impl this;
}
Now, other functions may also exploit such a shortcut. How do you plan
to implement that?
Actually, by adopting another idiom, it can be done:
struct Ref(Impl) {
private Impl* _impl;
@property ref Impl impl() {
return *(_impl = (_impl ? _impl : new Impl));
}
alias impl this;
auto opDispatch(string name, T...)(T args) {
mixin("return impl."~name~"(_impl,args);");
}
}
struct ExampleImpl {
static int length(ExampleImpl* that) {
return that ? *that.actualLength : 0;
}
int actualLength( ) {
return 42;
}
}
Of course, I did not say it was a good idiom. :p
I do something similar with RefCounted. There are problems - you need to
know in advance which functions you can implement on a null container
(empty and length are obvious candidates, but there could be others).
Andrei
