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

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2007-12-11 11:45:30

David Abrahams skrev:
> on Fri Dec 07 2007, Ion Gaztañaga <igaztanaga-AT-gmail.com> wrote:
> 
>> Thorsten Ottosen wrote:

>>> 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"

>> 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.

Right, but this is often a double-edged sword. Think of problems with 
allowing std::list<T>::size() to be O(n).

What is flexibility for the implementer is often a synonym for 
non-portability for the user. As a library implementer I have to be 
conservative w.r.t. the exceotion-safety guarantees given which means 
the additional guarantees given by some implementations cannot be taken 
advantage of. Of course, I might take advantage of them in non-generic 
code, but then at the expense of being non-portable.

-Thorsten