Re: How does std::set implement iterator through red black tree?

From:
Fei Liu <feiliu@aepnetworks.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 28 Jun 2007 13:49:49 -0400
Message-ID:
<f60sa8$tvf$1@aioe.org>
Ole Nielsby wrote:

Fei Liu <feiliu@aepnetworks.com> wrote:

This is a little more advanced topic. I am trying to understand why erase
on a set implemented through red black tree only invalidates the iterator
being erased? It'd seem to me when the RBTree needs to be re-balanced, the
ordering of iteration may change...What gives?


It works because set iterators are dumb. An iterator is just a pointer
to a node; it doesn't remember how it got there and has no plan where
to go next. When incremented, it will check the links to the surronding
nodes and act accordingly; and this will get you to the node with the
next higher key, no matter how the tree is balanced, as long as it is in
a well-defined state.

If one thread iterates while another is rebalancing, things might go
wrong because the tree may be in an interim ill-defined state (think
of a monkey jumping towards a branch that suddenly swivels to the
other side of the trunk while the poor animal is in mid air). So you
need to synchronize if several threads use a std::set: simultaneously.


Thanks,

What about the unordered set or map? How is iteration defined for this
type of containers?

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