Fragile ABI

[Paulo Pinto]

On Tuesday, 21 August 2012 at 15:15:02 UTC, David Piepgrass wrote:
> 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.

Even .NET with his goal of supporting multiple languages has the
CLS as the common set of datatypes and OO concepts to support 
across .NET
languages.

Given that OO has so many types of possible implementations, it is
hard to implement an ABI that works across multiple languages.

Lets see how the improved COM (WinRT) turns out to be.

--
Paulo