|  | 
Installs a handler into the slot, constructing the new object directly.
template< typename CancellationHandler, typename... Args> CancellationHandler & emplace( Args &&... args);
          Destroys any existing handler in the slot, then installs the new handler,
          constructing it with the supplied args.
        
The handler is a function object to be called when the signal is emitted. The signature of the handler must be
void handler(asio::cancellation_type_t);
                Arguments to be passed to the CancellationHandler
                object's constructor.
              
A reference to the newly installed handler.
          Handlers installed into the slot via emplace
          are not required to be copy constructible or move constructible.