Aquivalent References as in C++?

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Hi,

I have a few questions about D and could use some help. For
instance, how can i rebuild such a behavior?

class Foo {
public:
Foo(const Bar& b) {

At C++ you can ensure that a reference is requested and must not
null.
Nevertheless lvalues are functional, which means Foo(Bar(42))
would operate just as good as
Bar b(42);
Foo(b);

The same procedure doesn't seem to operate with const ref,
instead you can only use ref. But in this case Ivalues doesn't
operate.
Because of the fact that in D all objects are indication of
references I thought that I simply didn't need a storage class.
But then I recognized that at my D equivalent…

class Foo {
public:
this(Bar b) {


…null can also be passed.

That case I would like to prevent, but at the same time allow
Ivalues. How does that work in D classes?
As far as I know this operates with structs, but shouldn't it be
possible with classes and objects too?

Would be really nice if anyone had an advice for me.