[boost] Unordered associative containers and swap's exception safety

From: Daniel James (daniel_at_[hidden])
Date: 2005-02-11 04:45:20

Hello everyone,

I've got an exception safety problem implementing the hash containers. 
The draft standard says (from 6.3.1.1):

'For unordered associative containers, no swap function throws an 
exception unless that exception is thrown by the copy constructor or 
copy assignment operator of the container's Hash or [equality] Pred 
object (if any).'

This is a problem, because for the container to be valid, its hash 
buckets, equality predicate and hash function need to consistent. Since 
both the hash and predicate can throw, this is impossible (or hard?) to 
achieve. If swapping/copying the first one succeeds, and the second one 
throws they won't match. Any ideas on what I should do?

I don't think this can cause any major problems, like memory leaks or 
access to unowned memory, but it can cause unpredictable behaviour - 
since equal keys can end up in different buckets. Basically the 
invariant ends up totally broken.

The only solution that I've come up with, is to store the two function 
objects in a seperate structure, allocated on the heap, so that only 
pointers have to be swapped. But this seems unfortunate since most 
function objects won't throw.

This problem also applies to the copy assignment operator.

Daniel