#### async_write_at (3 of 4 overloads)

Start an asynchronous operation to write all of the supplied data at the specified offset.

    template<
        typename AsyncRandomAccessWriteDevice,
        typename Allocator,
        typename WriteToken = default_completion_token_t<        typename AsyncRandomAccessWriteDevice::executor_type>>
    DEDUCED async_write_at(
        AsyncRandomAccessWriteDevice & d,
        uint64_t offset,
        basic_streambuf< Allocator > & b,
        WriteToken && token = default_completion_token_t< typename AsyncRandomAccessWriteDevice::executor_type >(),
        constraint_t< !is_completion_condition< WriteToken >::value >  = 0);

This function is used to asynchronously write a certain number of bytes of data to a random access device at a specified offset. It is an initiating function for an asynchronous operation, and always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- All of the data in the supplied `basic_streambuf` has been written.
- An error occurred.

This operation is implemented in terms of zero or more calls to the device's async_write_some_at function, and is known as a composed operation. The program must ensure that the device performs no overlapping write operations (such as async_write_at, the device's async_write_some_at function, or any other composed operations that perform writes) until this operation completes. Operations are overlapping if the regions defined by their offsets, and the numbers of bytes to write, intersect.

###### Parameters

d

  The device to which the data is to be written. The type must support the AsyncRandomAccessWriteDevice concept.

offset

  The offset at which the data will be written.

b

  A `basic_streambuf` object from which data will be written. Ownership of the streambuf is retained by the caller, which must guarantee that it remains valid until the completion handler is called.

token

  The completion token that will be used to produce a completion handler, which will be called when the write completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        // Result of operation.
        const asio::error_code& error,

        // Number of bytes written from the buffers. If an error
        // occurred, this will be less than the sum of the buffer sizes.
        std::size_t bytes_transferred
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `async_immediate`.

###### Completion Signature

    void(asio::error_code, std::size_t)

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`

if they are also supported by the `AsyncRandomAccessWriteDevice` type's async_write_some_at operation.