Re: question about iterating hash_map

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

class{
public:
  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

add
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.