Construct a basic_stream_file
on an existing
native file.
basic_stream_file( const executor_type & ex, const native_handle_type & native_file);
This constructor initialises a stream file object to hold an existing native file.
The I/O executor that the file will use, by default, to dispatch handlers for any asynchronous operations performed on the file.
The new underlying file implementation.
Thrown on failure.