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Subject: Re: [boost] [review][constrained_value] Review of Constrained ValueLibrary begins today
From: Gordon Woodhull (gordon_at_[hidden])
Date: 2008-12-08 22:26:40

Replying to a day's messages in my own order. :-)

On Dec 8, 2008, at 3:14 PM, Paul A. Bristow wrote:
> Gordon wrote
>>> Also it might be worthwhile to point out that shared runtime mutable
>>> constraints with no overhead per constrained object are possible,
>>> using static members. (Or maybe that's too weird.) I might
>>> implement epsilon this way, as my impression is that
>>> numeric_limits<>::epsilon() is not useful for this purpose.
>> Of course I won't use this technique but something more like what's
>> described in "Compile-time fixed bounds" - don't know what I was
>> thinking. Runtime modification of epsilon is just silly.
> I'd just like to point out that there are more than the 'near-epilson
> computational noise' reasons, discussed so far, why it is useful to
> be able
> to make 'fuzzier' floating-point comparisons.

Yes, I am now convinced that all three use cases are valid in
different situations - there are times when you would want an epsilon
paired with each value, there are times when you want a class of
floats which are associated with a single runtime epsilon, and there
are times when epsilon can be chosen at compile time with a policy.

My point earlier was simply that the Boost.Test predicates won't work
out of the box because they require epsilon as a runtime argument -
epsilon needs to be built into the predicate in one of the three ways
in order to make it STL compatible.

It is interesting that size of predicates is not usually an issue
because they are passed to algorithms or bigger containers - since
constrained_value is a container of one suddenly it matters a lot.

Robert wrote:
> If an "exact floating point" type could be provided (out of scope of
> this library), being a wrapper for float/double and making sure that
> its
> underlying value is always truncated, you could perform comparisons
> (and all the
> other operations) that are repeatable, without the possibility that
> a comparison
> that once succeeded will later fail. Does it sound sensible?

I consider this a quick-and-dirty solution which would probably work
for a lot of situations. It does sound like it would be consistent;
however you still have the regular-old problem with floats, that
they're almost never equal except once in a blue moon.

I hope to find a way to implement it without assembly.

Kim Barrett wrote:
> inline double exact(double x) {
> struct { volatile double x; } xx = { x };
> return xx.x;
> }
> The idea is to force the value to make a round trip through a memory
> location of the "correct" size. The use of volatile should prevent
> the compiler from optimizing away the trip through memory.

Interesting! I still don't trust the compiler not to optimize it
away, but it's definitely worth a try.

Robert wrote:
> The "delta" (difference between extended and truncated value) may
> have a very big value for big numbers and very small for small ones,
> so epsilon should rather be scaled according to the magnitude of
> compared numbers.

The Knuth method implemented by Boost.Test multiplies epsilon by each
of the values and compares the difference in the values with the
result, so avoiding the scale problem.

Robert wrote:
> Thorsten wrote:
>> I totally disagree. People have to deal with floats anyway. That is a
>> seperate issue. The advice should be removed IMO, and
>> bounded_float provided.
> It should be provided, but Boost should first include some set of
> mechanisms to
> deal with the FP issues. They are too general to be implemented
> within this
> library and they are not tightly coupled with the concept of
> constrained types.
> I see this as an analogy to arithmetic overflows prevention, which
> is also too
> general and too orthogonal to this library.

Yes, floating point predicates should be a separate library, and the
floating point FUD should be removed from the documentation as well.

My yes vote is still conditional on your cooperation with us on
letting floating points work, because the library would not be useful
to me otherwise. I am now regretting that I didn't look at the code,
because I didn't know that there were assertions testing the
invariants in a different way from the predicates. Is this only in
the bounded part of the library?

I thought the value was tested after every change using the predicate,
and then called the error policy if the predicate failed. That's the
way I think it should be - you don't need to add any extra invariant
value to that, it's just perfect.

Like Stjepan said:
> As far as "if the test guarantees...the
> library guarantees...", it should be no more complicated to understand
> that "if X is thread-safe then something<X> is thread-safe", or
> something similar regarding exception safety.

This is the magic of C++, that templates can be used in unforeseen
ways because they take on the qualities of their arguments.

Generally I don't think a library should assert on any user input -
even an invalid/inconsistent predicate! I figured this was the point
of having an error policy - everyone has their own idea how they want
to handle errors. Assertions are forbidden in a lot of corporate

<joke>Perhaps there should be an predicate consistency policy.</joke>

I will look at the code tomorrow. Two last points:

1. I also like the monitored values use case and hope that you will
take the time to consider it before submitting a "final" version.
(Libraries are never truly finished.)

2. You don't have to worry about NaN - users can choose a predicate
that's appropriate for their application. Personally I would always
use a predicate that consistently rejects NaN, because I'd want the
error pointed out to me ASAP. But that should be implemented as a
separate check that is combined in, e.g. using std::logical_and<> (if
that hasn't been superceded by something in Boost).

I guess we have to wait for decltype to be able to use lambda
expressions as predicates here?


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