Question

If I change an element of an std::set, for example, through an iterator, I know it is not “reinserted” or “resorted”, but is there any mention of if it triggers undefined behavior? For example, I would imagine insertions would screw up. Is there any mention of specifically what happens?

Answer 1

You should not edit the values stored in the set directly. I copied this from MSDN documentation which is somewhat authoritative:

The STL container class set is used
for the storage and retrieval of data
from a collection in which the values
of the elements contained are unique
and serve as the key values according
to which the data is automatically
ordered. The value of an element in a
set may not be changed directly.
Instead, you must delete old values
and insert elements with new values.

Why this is is pretty easy to understand. The set implementation will have no way of knowing you have modified the value behind its back. The normal implementation is a red-black tree. Having changed the value, the position in the tree for that instance will be wrong. You would expect to see all manner of wrong behaviour, such as exists queries returning the wrong result on account of the search going down the wrong branch of the tree.