Re: [boost] Re: Formal Review for Interval Library beginning

From: Douglas Gregor (gregod_at_[hidden])
Date: 2002-09-02 21:43:05

On Monday 02 September 2002 02:02 am, Guillaume Melquiond wrote:
> > It's semantics correspond to the default Comparison policy
> > (compare_certainly).
>
> If I clearly understand what you mean, I need to disagree. The
> compare_certainly policy has been conceived to return a boolean value (the
> comparisons only answer "true" when the result is precisely "true").

I mispoke. If the return value were tribool then comparisons would act 
similarly to those with bool return values and the compare_certainly policy 
in boolean contexts, because with something like:

  interval<int> i,j; // init i, j
  if (i < j) {
    // ...
  }

would act the same. tribool returns true in a boolean context when the value 
is definitely/certainly true. Unfortunately, this would act differently:

  interval<int> i,j; // init i, j
  if (!(i < j)) {
    // ...
  }

compare_certainly would enter the if when it is not certain that i < j, 
whereas tribool would enter the if when it is certain that !(i < j). 
Essentially, the effect of the policy disappears once the result is converted 
to a bool. So compare_certainly can't state that 'the result of a boolean 
expression involving a comparison is true if the expression is certainly 
true', but is required to state the weaker 'the result of a comparison is 
true if the comparison is certainly true'.

        Doug