#### async_read_at (3 of 4 overloads)

Start an asynchronous operation to read a certain amount of data at the specified offset.

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

This function is used to asynchronously read a certain number of bytes of data from a random access device at the 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:

- An error occurred.

This operation is implemented in terms of zero or more calls to the device's async_read_some_at function.

###### Parameters

d

  The device from which the data is to be read. The type must support the AsyncRandomAccessReadDevice concept.

offset

  The offset at which the data will be read.

b

  A `basic_streambuf` object into which the data will be read. 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 read 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 copied into the buffers. If an error
        // occurred, this will be the number of bytes successfully
        // transferred prior to the error.
        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)

###### Remarks

This overload is equivalent to calling:

    asio::async_read_at(
       d, 42, b,
       asio::transfer_all(),
       handler);

###### 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 `AsyncRandomAccessReadDevice` type's async_read_some_at operation.