This D2 program shows that you can't test if a class reference wrapped with Rebindable is null:


import std.typecons;
const class Foo {}
void main() {
    auto a = Rebindable!Foo(new Foo);
    a = new Foo;
    assert(a !is null); // err
}


DMD 2.050 generates:
test.d(6): Error: incompatible types for ((a) !is (null)): 'Rebindable!(const(Foo))' and 'void*'


Is this the currently correct way to do it? (It works):

assert(a.get() !is null); // OK

I have seen the get() method is not documented on the site, so is that a temporary limitation caused by the unfinished "alis this" implementation?

Bye and thank you,
bearophile

On Sunday 21 November 2010 16:19:27 bearophile wrote:
> This D2 program shows that you can't test if a class reference wrapped with Rebindable is null:
> 
> 
> import std.typecons;
> const class Foo {}
> void main() {
>     auto a = Rebindable!Foo(new Foo);
>     a = new Foo;
>     assert(a !is null); // err
> }
> 
> 
> DMD 2.050 generates:
> test.d(6): Error: incompatible types for ((a) !is (null)):
> 'Rebindable!(const(Foo))' and 'void*'
> 
> 
> Is this the currently correct way to do it? (It works):
> 
> assert(a.get() !is null); // OK
> 
> I have seen the get() method is not documented on the site, so is that a temporary limitation caused by the unfinished "alis this" implementation?
> 
> Bye and thank you,
> bearophile

(a.get !is null) works. Why it's not documented, I don't know, and whether (a !is null) will ever be able to work with alias this, I don't know. I just know that (a.get !is null) currently works.

- Jonathan M Davis