Very newbie question coming up :)

How does D mark null values for classes?
`c is null` returns true, but `&c` isn't 0.
So how does D know `c is null`?

class C {}
C c;
assert(c is null);
assert(cast(size_t)&c == 0); // fails.

On Tuesday, 14 May 2013 at 12:18:20 UTC, simendsjo wrote:
> Very newbie question coming up :)
>
> How does D mark null values for classes?
> `c is null` returns true, but `&c` isn't 0.
> So how does D know `c is null`?
>
> class C {}
> C c;
> assert(c is null);
> assert(cast(size_t)&c == 0); // fails.

That's the address of the reference. If you want the address of the object:

assert(cast(size_t)cast(void*)c == 0); // passes

On Tuesday, 14 May 2013 at 12:18:20 UTC, simendsjo wrote:
> Very newbie question coming up :)
>
> How does D mark null values for classes?
> `c is null` returns true, but `&c` isn't 0.
> So how does D know `c is null`?
>
> class C {}
> C c;
> assert(c is null);
> assert(cast(size_t)&c == 0); // fails.

"&c" is address of reference, no class instance. I don't know if there is a way to get a pointer to class instance in D but I am not aware of one.

On Tuesday, 14 May 2013 at 12:20:00 UTC, Andrej Mitrovic wrote:
> On Tuesday, 14 May 2013 at 12:18:20 UTC, simendsjo wrote:
>> Very newbie question coming up :)
>>
>> How does D mark null values for classes?
>> `c is null` returns true, but `&c` isn't 0.
>> So how does D know `c is null`?
>>
>> class C {}
>> C c;
>> assert(c is null);
>> assert(cast(size_t)&c == 0); // fails.
>
> That's the address of the reference. If you want the address of the object:
>
> assert(cast(size_t)cast(void*)c == 0); // passes

Ah. Thanks to you both. Now I have some strange bug where somewhere in my code (probably using alias for passing symbols to templates), it get's a "wrong" reference...

Am 14.05.2013 14:24, schrieb Dicebot:
> On Tuesday, 14 May 2013 at 12:18:20 UTC, simendsjo wrote:
>> Very newbie question coming up :)
>>
>> How does D mark null values for classes?
>> `c is null` returns true, but `&c` isn't 0.
>> So how does D know `c is null`?
>>
>> class C {}
>> C c;
>> assert(c is null);
>> assert(cast(size_t)&c == 0); // fails.
> 
> "&c" is address of reference, no class instance. I don't know if there
> is a way to get a pointer to class instance in D but I am not aware of one.

A simple cast to void* should do it: cast(void*)c

On Wednesday, 15 May 2013 at 10:31:29 UTC, David wrote:
>> "&c" is address of reference, no class instance. I don't know if there
>> is a way to get a pointer to class instance in D but I am not aware of one.
>
> A simple cast to void* should do it: cast(void*)c

Is it actually defined somewhere in spec?

On 05/15/13 13:04, Dicebot wrote:
> On Wednesday, 15 May 2013 at 10:31:29 UTC, David wrote:
>>> "&c" is address of reference, no class instance. I don't know if there
>>> is a way to get a pointer to class instance in D but I am not aware of one.
>>
>> A simple cast to void* should do it: cast(void*)c

   class C { auto opCast(T:void*)() { return null; } }

So - no - a simple cast to void* won't always work, as the op can be overloaded, even if only by accident.

See also:
http://forum.dlang.org/thread/mutvhrrdfzunbrhmvztp@forum.dlang.org#post-mailman.2432.1354406246.5162.digitalmars-d-learn:40puremagic.com
and
http://d.puremagic.com/issues/show_bug.cgi?id=8545


artur

On Wed, 15 May 2013 11:08:33 -0400, Artur Skawina <art.08.09@gmail.com> wrote:

> On 05/15/13 13:04, Dicebot wrote:
>> On Wednesday, 15 May 2013 at 10:31:29 UTC, David wrote:
>>>> "&c" is address of reference, no class instance. I don't know if there
>>>> is a way to get a pointer to class instance in D but I am not aware of one.
>>>
>>> A simple cast to void* should do it: cast(void*)c
>
>    class C { auto opCast(T:void*)() { return null; } }
>
> So - no - a simple cast to void* won't always work, as the op can be
> overloaded, even if only by accident.

You can insert a cast to Object first.

-Steve

>> You can insert a cast to Object first.

doesn't seem to work.
I'm trying to write a generic solution but having an innocent 'T opCast(T :
int)() { return 1; }' breaks it, see Error below.
doesn't seem to work.

This seems to call for a compiler solution?


 import std.stdio;
import std.conv;


void*AddressOf(T)(T a) if(is(T==class)){
    return cast(void*)cast(Object) a;//Error: template instance opCast!(
Object) opCast!(Object) does not match template declaration opCast(T : int)(
)
}
T*AddressOf(T)(ref T a) if(!is(T==class)){
    return &a;
}
class A{
    int x;
    T opCast(T : int)() { return 1; }
//    int opCast(T:void*)() { return 1; } //won't work either
}
struct B{

}
void main(){
    A a;
    writeln(AddressOf(a));
    a = new A;
    writeln(AddressOf(a));

    B b;
    writeln(AddressOf(b));
    B*b2;
    writeln(AddressOf(*b2));
//    writeln(AddressOf(B.init)); //waiting for rvalue ref, DIP39
}


On Wed, May 15, 2013 at 11:30 AM, Steven Schveighoffer <schveiguy@yahoo.com>wrote:

> On Wed, 15 May 2013 11:08:33 -0400, Artur Skawina <art.08.09@gmail.com> wrote:
>
>  On 05/15/13 13:04, Dicebot wrote:
>>
>>> On Wednesday, 15 May 2013 at 10:31:29 UTC, David wrote:
>>>
>>>> "&c" is address of reference, no class instance. I don't know if there
>>>>> is a way to get a pointer to class instance in D but I am not aware of one.
>>>>>
>>>>
>>>> A simple cast to void* should do it: cast(void*)c
>>>>
>>>
>>    class C { auto opCast(T:void*)() { return null; } }
>>
>> So - no - a simple cast to void* won't always work, as the op can be overloaded, even if only by accident.
>>
>
> You can insert a cast to Object first.
>
> -Steve
>

15-May-2013 23:17, Timothee Cour пишет:
>  >> You can insert a cast to Object first.
>
> doesn't seem to work.
> I'm trying to write a generic solution but having an innocent
> 'TopCast(T:int)(){return1;}' breaks it, see Error below.
> doesn't seem to work.
>
> This seems to call for a compiler solution?

Won't this work?
*cast(void**)&object
>
> importstd.stdio;
> importstd.conv;
>
>
> void*AddressOf(T)(Ta)if(is(T==class)){
> returncast(void*)cast(Object)a;//Error:templateinstanceopCast!(Object)opCast!(Object)doesnotmatchtemplatedeclarationopCast(T:int)()
> }
> T*AddressOf(T)(refTa)if(!is(T==class)){
> return&a;
> }
> classA{
> intx;
> TopCast(T:int)(){return1;}
> //intopCast(T:void*)(){return1;} //won't work either
> }
> structB{
>
> }
> voidmain(){
> Aa;
> writeln(AddressOf(a));
> a=newA;
> writeln(AddressOf(a));
>
> Bb;
> writeln(AddressOf(b));
> B*b2;
> writeln(AddressOf(*b2));
> //writeln(AddressOf(B.init));//waitingforrvalueref,DIP39
> }
>
>
> On Wed, May 15, 2013 at 11:30 AM, Steven Schveighoffer
> <schveiguy@yahoo.com <mailto:schveiguy@yahoo.com>> wrote:
>
>     On Wed, 15 May 2013 11:08:33 -0400, Artur Skawina
>     <art.08.09@gmail.com <mailto:art.08.09@gmail.com>> wrote:
>
>         On 05/15/13 13:04, Dicebot wrote:
>
>             On Wednesday, 15 May 2013 at 10:31:29 UTC, David wrote:
>
>                     "&c" is address of reference, no class instance. I
>                     don't know if there
>                     is a way to get a pointer to class instance in D but
>                     I am not aware of one.
>
>
>                 A simple cast to void* should do it: cast(void*)c
>
>
>             class C { auto opCast(T:void*)() { return null; } }
>
>         So - no - a simple cast to void* won't always work, as the op can be
>         overloaded, even if only by accident.
>
>
>     You can insert a cast to Object first.
>
>     -Steve
>
>


-- 
Dmitry Olshansky