Return a copy of the value associated with the specified key in a given
DataType at(KeyType const& key);
A copy of the value associated with
at the point of the call.
std::out_of_range if the specified
key is not present
Any exceptions thrown by the copy constructor or move constructor
exception thrown by the application of the
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.