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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'.


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