Re: [boost] [review] Review of Unordered library begins today December 7

From: David Abrahams (dave_at_[hidden])
Date: 2007-12-11 11:00:11

on Fri Dec 07 2007, Ion Gaztañaga <igaztanaga-AT-gmail.com> wrote:

> Thorsten Ottosen wrote:
>> I vote to accept this library. The documentation is very nice.
>
> Thanks for the vote.
>
>> One thing puzzled me though:
>> (see http://igaztanaga.drivehq.com/unordered/unordered/comparison.html)
>> 
>> "The containers hash or predicate function can throw exceptions from erase"
>> 
>> Under what cicumstances can/should/would I construct a ahsh or predicate 
>> that throws? I mean, the hash computes an integer, and comparisons reads 
>> data to determine its answer.
>> 
>> This migth be what the standard requires, but it certainly seems 
>> non-intuitive. Why not just require that neither hash nor predicates can 
>> throw?
>
> The same could be said about Predicate for ordered containers like 
> std::set/map. I haven't seen a practical case where the comparison could 
> throw. But standard library experts should answer this better than me.
>
> Certainly, Daniel has tried to achieve strong exception guarantees and 
> the implementation becomes quite complicated if comparison/hash throws. 
> Double buffering and other tricks are needed. I think this is a very 
> good question both for boosters and people from the LWG.

When specifying requirements in the standard, we (the LWG) don't like
to constrain implementations or users if possible.  I don't see any
reason to forbid a throwing comparison or hash.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com