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Wrap a function object in a packaged task.
template< typename Function> unspecified operator()( Function && f) const;
            The package function
            is used to adapt a function object as a packaged task. When this adapter
            is passed as a completion token to an asynchronous operation, the result
            of the function object is retuned via a std::future.
          
std::future<std::size_t> fut = my_socket.async_read_some(buffer, use_future([](asio::error_code ec, std::size_t n) { return ec ? 0 : n; })); ... std::size_t n = fut.get();