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digitalmars.D - interfaces ABI

reply =?ISO-8859-1?Q?Lu=EDs_Marques?= <luismarques gmail.com> writes:
Hello,

The interfaces ABI is marked "TBD". Could this be filled, please?

I found the following surprising, regarding the ABI:

An object only has a pointer to the vtable and a monitor, the rest being 
the actual data ("non-static members"). At the vtable's offset 0 we find 
a pointer to the classinfo. And the classinfo has an array of 
interfaces. So when I add an interface, I would assume the objects to 
remain equal in size. The classinfo would get a bigger array of 
interfaces, and perhaps the vtable would also get bigger. But since no 
non-static member were added to the class, I expected the object size to 
be equal. Yet, it gets bigger.

--
Luís
Mar 19 2007
parent BCS <BCS pathlink.com> writes:
Luís Marques wrote:
 Hello,
 
 The interfaces ABI is marked "TBD". Could this be filled, please?
 
 I found the following surprising, regarding the ABI:
 
 An object only has a pointer to the vtable and a monitor, the rest being 
 the actual data ("non-static members"). At the vtable's offset 0 we find 
 a pointer to the classinfo. And the classinfo has an array of 
 interfaces. So when I add an interface, I would assume the objects to 
 remain equal in size. The classinfo would get a bigger array of 
 interfaces, and perhaps the vtable would also get bigger. But since no 
 non-static member were added to the class, I expected the object size to 
 be equal. Yet, it gets bigger.
 
 -- 
 Luís

IIRC interface are implemented by adding a pointer to the interfaces v-tbl to the object, then when a object is cast to an interface the interface ends up as a pointer to /it's/ v-tbl in the object. The reason for this is that the interface's v-tbl includes an offset to the start of the object. When an interface is used to make a call it ends up looking something like this. I i; ((*i)[member_offset]) (i+(*i)[v_tbl_offset], /** args **/ ) Why the v-tbls are in the classinfo? I don't know. What I would like to see is interfaces become a context/v-tbl pair (sort of like arrays being a pointer/length pair). Then, when an interface is used to make a call it would end up looking something like this. I i; (i.vtbl[member_offset]) (i.context, /** args **/ ) this would free classes from having to carry around a v-tbl pointer and also let other things like struct and functions implement interfaces.
Mar 19 2007