• realhet (16/16) Jan 16 2020 Hello,
• Adam D. Ruppe (5/8) Jan 16 2020 I'm pretty sure the only fields in there are pointer to vtable
• Petar Kirov [ZombineDev] (2/10) Jan 16 2020 Alternatively, the class can be marked as extern(C++).
• realhet (24/32) Jan 16 2020 Thank you both for the hints!
• kinke (16/26) Jan 16 2020 The 3 latter points can be trivially worked around via extern(D):

realhet <real_het hotmail.com> writes:

Hello,
I'm try to figure out the contents od the base class instance in 
D, LDC Windows 64bit.
I'm sure that there is a 8 byte VMT pointer.
But unable to fund information about the remaining 8 byte. I 
guess it must be a unique 8byte (void* ?) identifier that is 
useful for Monitor. In a video someone mentioned this as 'magic'.

Is there a documentation about that 'magic' field? I'd love to 
use some bits in it if I'm sure about the consequences.

I have a really small object, only 32 bytes. At this point if I 
want to add a flag bit I have 3 choices:
- add a new field to the class -> effectively the instanceSize 
will grow by 50% (16bytes with alignment)
- compress the current 16bytes of data. -> eats more cpu
- Hide it somewhere 'magically'.

Thank You in advance!

Adam D. Ruppe <destructionator gmail.com> writes:

On Thursday, 16 January 2020 at 14:30:04 UTC, realhet wrote:
 Is there a documentation about that 'magic' field?

I'm pretty sure the only fields in there are pointer to vtable 
and pointer to monitor object...

 I have a really small object, only 32 bytes. At this point if I 
 want to add a flag bit I have 3 choices:

Do you need virtual functions? If not, you could probably just 
make a struct instead.

Petar Kirov [ZombineDev] <petar.p.kirov gmail.com> writes:

On Thursday, 16 January 2020 at 14:32:24 UTC, Adam D. Ruppe wrote:
 On Thursday, 16 January 2020 at 14:30:04 UTC, realhet wrote:
 Is there a documentation about that 'magic' field?

 I'm pretty sure the only fields in there are pointer to vtable 
 and pointer to monitor object...

 I have a really small object, only 32 bytes. At this point if 
 I want to add a flag bit I have 3 choices:

 Do you need virtual functions? If not, you could probably just 
 make a struct instead.

Alternatively, the class can be marked as extern(C++).

realhet <real_het hotmail.com> writes:

On Thursday, 16 January 2020 at 14:32:24 UTC, Adam D. Ruppe wrote:
 On Thursday, 16 January 2020 at 14:30:04 UTC, realhet wrote:
 Is there a documentation about that 'magic' field?

 I'm pretty sure the only fields in there are pointer to vtable 
 and pointer to monitor object...

 I have a really small object, only 32 bytes. At this point if 
 I want to add a flag bit I have 3 choices:

 Do you need virtual functions? If not, you could probably just 
 make a struct instead.

Thank you both for the hints!

Yes I need virtual functions (this is the base class of my layout 
system: it can be a letter, a picture, or a paragraph of text.).

I've tried extern(C++) and it went down 8 bytes.

But the I tried the following:
synchronized(obj){ ... }
Compiles without a problem. Hmm...

I think I will use extern(C++) and later when I will have a big 
system to test, I will benchmark it. I'm sure that Monitor 
functionality is not needed for these objects, so that extra 8 
bytes will worth it.

Update:
- All of the child classes needed to be marked with extern(C++)
- static class members are not supported, only __gshared static.
- passing a delegate to a constructor of this class expects a 
(extern(C++) delegate) too.
- Internal compiler error: string[string] can not be mapped to C++

So extern(C++) is not good in the current case.

I will try to use that 8 byte magic illegally, and will see if it 
is unstable or not.
I will not do any synchronization, but I think the GC will crash 
upon releasing these objects.

Thx for the help!

kinke <kinke gmx.net> writes:

On Thursday, 16 January 2020 at 15:28:06 UTC, realhet wrote:
 Update:
 - All of the child classes needed to be marked with extern(C++)
 - static class members are not supported, only __gshared static.
 - passing a delegate to a constructor of this class expects a 
 (extern(C++) delegate) too.
 - Internal compiler error: string[string] can not be mapped to 
 C++

 So extern(C++) is not good in the current case.

The 3 latter points can be trivially worked around via extern(D):

extern(C++) class C
{
     extern(D):
     static int tlsGlobal;
     this(void delegate()) {}
     void foo(string[string] aa) {}
}

void main()
{
     C.tlsGlobal = 123;
     auto c = new C(() {});
     c.foo(null);
}

 I will not do any synchronization, but I think the GC will 
 crash upon releasing these objects.

Very likely.