Obtain the number of items stored in the map.
size_type size() const;
The number of key/value pairs store in
may be called concurrently with any other member function of
jss::concurrent_map on a given instance
(except for the constructor and destructor) without external synchronization.
returns the count at the point of the call. Operations on other
threads may insert additional elements or remove existing ones
between a call to
size() and the caller making use of
the returned size.