STL sorted set where the conditions of order may change

Question

I have a C++ STL set with a custom ordering defined.

The idea was that when items get added to the set, they're naturally ordered as I want them.

However, what I've just realised is that the ordering predicate can change as time goes by.

Presumably, the items in the set will then no longer be in order.

So two questions really:

1. Is it harmful that the items would then be out of order? Am I right in saying that the worst that can happen is that new entries may get put into the wrong place (which actually I can live with). Or, could this cause crashes, lost entries etc?

2. Is there a way to "refresh" the ordering of the set? You can't seem to use std::sort() on a set. The best I can come up with is dumping out the contents to a temp container and re-add them.

Any ideas?

Thanks,

John

Answer 1

set uses the ordering to lookup items. If you would insert N items according to ordering1 and insert an item according to ordering2, the set cannot find out if the item is already in.

It will violate the class invariant that every item is in there only once.

So it does harm.

Answer 2

The only safe way to do this with the STL is to create a new set with the changed predicate. For example you could do something like this when you needed to sort the set with a new predicate:

std::set<int> newset( oldset.begin(), oldset.end(), NewPred() );