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~io_context();
          On destruction, the io_context performs the following
          sequence of operations:
        
svc
              in the io_context
              set, in reverse order of the beginning of service object lifetime,
              performs svc->shutdown().
            io_context,
              or any associated strand, are destroyed.
            svc
              in the io_context
              set, in reverse order of the beginning of service object lifetime,
              performs delete static_cast<io_context::service*>(svc).
            
          The destruction sequence described above permits programs to simplify their
          resource management by using shared_ptr<>. Where an object's lifetime is
          tied to the lifetime of a connection (or some other sequence of asynchronous
          operations), a shared_ptr
          to the object would be bound into the handlers for all asynchronous operations
          associated with it. This works as follows:
        
shared_ptr references
              to the objects are destroyed.
            io_context function stop()
              is called to terminate any run() calls as soon as possible. The io_context
              destructor defined above destroys all handlers, causing all shared_ptr references to all connection
              objects to be destroyed.