Re: question about iterating hash_map

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Thu, 31 Jul 2008 13:58:46 GMT
On 2008-07-31 15:50, wrote:


I have a hash_map with string as key and an object pointer as value.
the object is like

  float a;
  float b;

I have two threads, one thread is keep updating the hash_map, add new
or grab the object and update the value. Another thread is retrieving
the object and read the value randomly.

It is ok if the reading thread get the stale value sometimes, but not
acceptable if reading corrupted value inside the object.

For performance concern, I do not want to lock the hash_map when two
threads are doing as

find by key and grab the object and update the value inside the object

I will put a mutex inside the object to make sure the read will not
get corrupted value. However, I am little concerned when one thread
doing the find on the key while another thread is adding a new item,
which will change the size of the hash_map.

In C#, if I do not lock a collection when doing iteration, other
action on the hash_map at the same time (as an add) may cause the
exception. I have a little concerned whether the same problem could
happen on C++ hash_map. I am using Redhat with gcc, on ext/hash_map.

Please note that hash_map is not part of the C++ language and as such it
is not topical in this group. What you should do is to read the
documentation for you implementation and see if inserting elements
invalidates iterators, for some containers it does not not. Also you
want to know if operations on the container are thread-safe or not.

If the operations are not thread-safe but insertions does not invalidate
iterators you only need a mutex around the insertion and around the find
operations, but not for the update.

Erik Wikstr??m

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