Daniel James skrev:
2009/7/14 Thorsten Ottosen <thorsten.ottosen@dezide.com>:
I think this a really bad idea to allocate in the default construtor. The user can always call rehash() on the empty container, just like when we call reserve() for vector.
The unordered containers are specified in a way that suggests memory will always be allocated for the buckets - and IIRC all the publicly available implementations do that.
Although it is possible to avoid the allocation when constructing empty containers (I think a while ago I said it wasn't, but I've worked out how since). A check needs to be added to pretty much every method to make sure that the buckets have been allocated which might have a performance hit and increase the executable size.
I thought a little about this. Do you think it would be possible to add a constructor like boost::unordered_xxx container( boost::dont_allocate_memory_tag() ); The postcondition for this constructor should only guarantee that - the object is swappable - the object is destructible - the object is assignable - the object is copyable Calling anything else should result in undefined behavior. ? -Thorsten