Determine whether or not the map is empty.
bool empty() const;
true if there are
no items 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.
The return value of
empty() is based on the contents of the
container at the point of the call. Operations on other threads
may insert additional elements or remove existing ones between
a call to
empty() and the caller making use of
the returned value.