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        A shutdown handler must meet the requirements for a handler.
        A value h of a shutdown handler
        class should work correctly in the expression h(ec),
        where ec is an lvalue of
        type const error_code.
      
A free function as a shutdown handler:
void shutdown_handler( const asio::error_code& ec) { ... }
A shutdown handler function object:
struct shutdown_handler { ... void operator()( const asio::error_code& ec) { ... } ... };
        A non-static class member function adapted to a shutdown handler using bind():
      
void my_class::shutdown_handler( const asio::error_code& ec) { ... } ... ssl_stream.async_shutdown( boost::bind(&my_class::shutdown_handler, this, asio::placeholders::error));