Re: Simple example on interfaces

["Randy Brukardt" <randy@rrsoftware.com>]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:rumtg6$j94$1@dont-email.me...
> On 1/25/21 5:08 PM, Mario Blunk wrote:
>> I'm trying to solve a problem of multiple inheritance. It seems to me 
>> that an interface could be the solution although the interface is still a 
>> mystery for me.
>
> "IMHO, Interfaces are worthless."
> Randy Brukardt

To qualify that a bit, they're worthless to me (and I suspect, most people). 
For me, at least, OOP's benefits are mainly found in implementation 
inheritance, which is not available for Interfaces. You have to use abstract 
types to get those benefits.

For a single program, an interface doesn't buy anything, because it is very 
unlikely that you'll have more than one implementation of the interface in 
use. (Think the queue interface in Annex A.) So using dispatching just adds 
complication but no benefit; most likely you'll statically bind everything 
anyway.

Which pretty much leaves reusuable code. Here, dispatching probably does 
have some benefit. But you can get similar benefits from generic units with 
formal derived type parameters. The problem is that interface dispatching is 
quite expensive (not just the indexing of single inheritance dispatching, 
but also some sort of lookup of the appropriate table). Whereas the generic 
solution does most of the binding at compile-time.

It may be my optimizer guru background, but indirect calls are pretty much 
unoptimizable. Ergo, the cost of dispatching is even worse than it appears 
on the surface, given that valuable optimizations like inlining, partial 
evaluation (currying), and all of the things that they enable aren't 
possible. So if the code performance matters, ultimately the interfaces will 
have to go. (Of course, if it *doesn't* matter, one shouldn't be warping a 
design for performance reasons. But it is *hard* to get rid of interfaces 
that are too expensive, so I think it makes most sense to be sparing with 
their use.)

Ultimately, I think one should only use interfaces IFF there is a clear 
reuse case where the substantial cost of dispatching is not a concern. For 
me, that is approximately never, but of course your mileage may vary.

                                               Randy.