Comments on: Gem #1: Limited Types in Ada 2005 — Limited Aggregates

By: Paul F. Pearson

Paul F. Pearson — Wed, 30 May 2007 14:07:20 +0000

Bob Duff said:

Note that my example used Unbounded_String, and then I turn around and say controlled types are expensive. Oh well, it’s just an example.

That actually helps answer my question – a feature can be “expensive”, but worth the expense (e.g. a Mercedes vs. a Pinto). In your example, the simplicity of the Unbounded_String helped keep the focus on the subject (in spite of my tangent).

By: Bob Duff

Bob Duff — Mon, 21 May 2007 13:56:04 +0000

Paul F. Pearson said, “Of course, this means that Unbounded_String will be used less often… :-)”

Yes, Unbounded_String is controlled and is therefore expensive. Whether it’s _too_ expensive can best be determined by measuring.

Note that my example used Unbounded_String, and then I turn around and say controlled types are expensive. Oh well, it’s just an example.

By: Paul F. Pearson

Paul F. Pearson — Fri, 18 May 2007 20:18:10 +0000

I forgot to comment on the topic:

This looks to be a *very* useful feature of Ada 2005. Thank you for pointing this out.

I look forward to the next “gem”.

By: Paul F. Pearson

Paul F. Pearson — Fri, 18 May 2007 20:16:27 +0000

Thank you, Franco.

That paper is *very* informative (as usual). I can see that there is quite a bit of overhead. While I was hoping to find an analysis quantifying the expense, I can see that my hope was a bit unrealistic.

I suppose that my design decisions related to controlled types will be made like other features – if my code needs to be fast, be *very* thoughtful about using controlled types.

Of course, this means that Unbounded_String will be used less often… :-)

By: Franco Gasperoni

Franco Gasperoni — Thu, 17 May 2007 14:17:37 +0000

“What is the cost of controlled types”, asks Paul.

The way controlled types are currently implemented in GNAT is to put each controlled object onto a finalization list so that Finalize can be called before the object gets destructed.

So each controlled object has two extra pointers to be doubly chained. So there is a space cost (for you to decide whether that is acceptable or not).

The runtime cost is adding/removing each controlled object onto the finalization list and calling initialize/adjust/finalize when appropriate.

More details are available in a paper that Cyrille, Gary, and I wrote several years ago. The paper is available at:

http://www.adacore.com/2001/01/26/the-gnat-implementation-of-controlled-types/

Enjoy :)

By: Paul F. Pearson

Paul F. Pearson — Wed, 16 May 2007 13:47:08 +0000

“[...]but controlled types are expensive”. I’ve heard this before, and I can believe it. However, has there been any analysis of *how* expensive? What is their cost compared to a custom implementation providing similar functionality (e.g. providing a “deep copy” procedure/function)? How can someone decide when a controlled type is “too expensive”?

For instance, when should I decide to *not* use Unbounded_String, since (at least in the GNAT versions I’ve seen) it uses a Controlled type?