Re: Ada Annoyances

[raph.amiard@gmail.com]

Le mercredi 21 juin 2017 18:12:53 UTC+2, Dmitry A. Kazakov a écrit :
> On 2017-06-21 16:52, pytho...@gmail.com wrote:
> > I definitely agree that performance is best measured, not guessed
> > about. However, the way I see it, this restriction is (at least for my
> > use case where I know I don't need tagged types) at best making the
> > compiler work much harder to see that it can devirtualize the function
> > calls and at worst making my code run slower with no benefit - other
> > than providing a slightly nicer syntax.
> 
> In Ada there is no penalty of calling a "method" vs. a "free" function. 
> None, whatsoever. This per language type system design and different 
> from C++.
> 
> I don't consider here the overhead of using any helper types/objects 
> and/or class-wide objects. That has nothing to do with the type under 
> consideration being tagged or not.
> 
> The penalty of a tagged type is only this:
> 
> 1. Space to keep the type tag
> 2. By-reference passing only, even if the object is small.
> 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de

Well that's really a strange way to put it: 

1. In Ada you dispatch via going through the class wide type. But if you do, then your call *will* go through a virtual table.

2. In C++ only virtual methods dispatch and go through the virtual table. The penalty on the type is exactly the same (by-ref passing and tag)

3. The ONLY thing that differs is the granularity of how you choose to dispatch or not. In C++, this choice is made by the method's implementer. In Ada, it is made at the call-site by every caller.

It could very well be argued that the C++ choice is a lot better. Most of the time, you never want to call a possibly dispatching method in without dispatching. Ada gives you a little bit more flexibility, at the cost of being much more error prone, because it's easy to make a non dispatching call instead of a dispatching one. Not a good point for Ada !