Re: Thread safety in set

Ulrich Eckhardt <>
Thu, 24 Jan 2008 12:44:37 +0100
tk wrote:

From MSDN Library reference "set Class", it says "Inserting elements
invalidates no iterators, and removing elements invalidates only those
iterators that had specifically pointed at the removed elements."

Does this mean that multiple threads inserting to a set and iterating a
set is OK, as long as there is no threading removing from it?

No. While inserting elements, internal data structures can well be in a
state that wouldn't allow reading from the set. It is only before and after
these operations that a consistent state exists. Further, and that is an
issue often ignored by programmers, thread safety is not only about
atomicity but also about visibility, in particular in systems with multiple

This seems contradicting to another referece "Thread Safety in the
Standard C++ Library" in MSDN Library:

"The container classes are vector, deque, list, queue, stack ,
priority_queue, valarray, map, multimap, set, multiset, basic_string,
bitset. For reads to the same object, the object is thread safe for
reading in the following scenarios:
From one thread at a time when no writers on other threads.
From many threads at a time when no writers on other threads.
For writes to the same object, the object is thread safe for writing from
one thread when no readers on other threads"

This is not contradicting the above. The above has nothing to do with
multithreading, it only documents required iterator invalidation rules from
the C++ standard.


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