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#### 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 WriteHandler>
    void-or-deduced async_write_at(
        AsyncRandomAccessWriteDevice & d,
        uint64_t offset,
        basic_streambuf< Allocator > & b,
        WriteHandler handler);

This function is used to asynchronously write a certain number of bytes of data to a random access device at a specified offset. The function call 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 handler is called.

handler

  The handler to be called when the write operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

      void handler(
        // Result of operation.
        const boost::system::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 handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `boost::asio::io_service::post()`.