#### ssl::stream::async_read_some

Start an asynchronous read.

    template<
        typename MutableBufferSequence,
        typename ReadToken = DEFAULT>
    DEDUCED async_read_some(
        const MutableBufferSequence & buffers,
        ReadToken && token = DEFAULT);

This function is used to asynchronously read one or more bytes of data from the stream. It is an initiating function for an asynchronous operation, and always returns immediately.

###### Parameters

buffers

  The buffers into which the data will be read. Although the buffers object may be copied as necessary, ownership of the underlying buffers is retained by the caller, which must guarantee that they remain 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(
        const asio::error_code& error, // Result of operation.
        std::size_t bytes_transferred // Number of bytes read.
      );

  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 `post`.

###### Completion Signature

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

###### Remarks

The async_read_some operation may not read all of the requested number of bytes. Consider using the `async_read` function if you need to ensure that the requested amount of data is read before the asynchronous operation completes.

###### 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 `Stream` type's `async_read_some` and `async_write_some` operations.