Re: bug or documentation missing boost posix_time time_duration not_a_date_time

From: Gavin Lambert (boost_at_[hidden])
Date: 2019-05-23 07:10:08

On 23/05/2019 17:37, Erik Thiele wrote:
>> not_a_date_time is like a NaN.  NaNs are weird like that.
> 
> i don't think we should mix floating point issues with the date_time
> library.

It's not implemented with an actual floating point NaN, but it is 
clearly intended to have equivalent behaviour.

https://github.com/boostorg/date_time/blob/develop/include/boost/date_time/int_adapter.hpp

See the above code.  Although interestingly unlike some floating point 
implementations, it looks like it does consider not_a_date_time == 
not_a_date_time.

But yes, it should either give not_a_date_time an actual consistent 
relative ordering or use partially_ordered instead of less_than_comparable.

> but i expect v0 and v1 to be equal because after sorting two
> vectors with the same contents must be equal. watch closely, v0 and v1
> have the same elements. they are just push_backed in a different order.
> so after sorting v0 and v1, they need to be equal, but aren't. this is
> because operator< is not a complete partial order here.

Ah, I missed that part.  But still, no.

Sorting only uses operator<, and those operators are implemented 
consistently with NaN-like behaviour (partial ordering).

In particular, it is *not* a "strict weak ordering", which is what 
std::sort assumes.

not_a_date_time has no specific order, therefore sort is allowed to put 
it wherever it likes; you can't make any assumptions about its position 
relative to other values.  It's not even required to group them together 
if they're spread out (although some sort algorithms might do that).

If you don't like that behaviour, you can pass your own different 
comparator to sort which does assign a strict ordering to the values. 
For example, you can declare that not_a_date_time should be treated as 
strictly less than any actual duration.

The bug you noticed in the <= and >= return values has no effect on sort.