Fragile ABI

[David Piepgrass]

On Monday, 20 August 2012 at 18:37:00 UTC, R Grocott wrote:
> On Monday, 20 August 2012 at 15:26:48 UTC, Kagamin wrote:
>> What you ask for sounds quite similar to COM composition with 
>> delegation.
>
> Would anybody mind linking to resources which describe COM 
> composition with delegation? It's been suggested twice in this 
> thread as an alternative way to develop a non-fragile API, but 
> anything related to COM is almost invisible to search engines 
> (even moreso than D itself).

There's nothing novel about COM except aggregation, and 
aggregation is just an implementation detail where a class 
pretends that it implements an interface but the calls to that 
interface go to another object, conceptually it's like "alias 
this" except that a dynamic cast (i.e. QueryInterface) is 
required to reach the second object:

http://msdn.microsoft.com/en-us/library/ms686558(v=vs.85)

For the most part COM sucks really bad: it is a very ordinary 
object-oriented ABI but without numerous features that we 
otherwise take for granted:

- In COM, you can't define static methods
- In COM, you can't overload functions
- In COM, constructors can't have arguments
- In COM, there are no fields, only properties
- In COM, class inheritance is not allowed (an interface IB can 
inherit from IA, but if you implement a class A that implements 
IA, you can't write a class B that derives from A and implements 
IB. In C++/ATL a template-based workaround is possible if A and B 
are in the same DLL.)

Moreover COM ABIs are fragile, in that there is almost zero 
support for adding or removing methods without either breaking 
everything or creating a new, independent, incompatible version 
(the only exception: you can safely add a method at the end of an 
interface, if you can be certain that no other interface inherits 
from it.)

Finally, it's Windows-only (although it has been reimplemented on 
Linux, e.g. for WINE) and modules must be registered in the 
Windows Registry.

I think the only reason we still use COM today is that, sadly, 
there is no other OO standard interoperable with all languages. 
C++ vtables are the closest competitor; I guess their fatal flaw 
is that there is no standard for memory management across C++ 
DLLs.