A read token is a completion token for completion signature void(error_code, size_t).

Examples

A free function as a read token:

void read_handler(
    const boost::system::error_code& ec,
    std::size_t bytes_transferred)
{
  ...
}

A read token function object:

struct read_handler
{
  ...
  void operator()(
      const boost::system::error_code& ec,
      std::size_t bytes_transferred)
  {
    ...
  }
  ...
};

A lambda as a read token:

socket.async_read_some(...,
    [](const boost::system::error_code& ec,
      std::size_t bytes_transferred)
    {
      ...
    });

A non-static class member function adapted to a read token using std::bind():

void my_class::read_handler(
    const boost::system::error_code& ec,
    std::size_t bytes_transferred)
{
  ...
}
...
socket.async_read_some(...,
    std::bind(&my_class::read_handler,
      this, std::placeholders::_1,
      std::placeholders::_2));

A non-static class member function adapted to a read token using boost::bind():

void my_class::read_handler(
    const boost::system::error_code& ec,
    std::size_t bytes_transferred)
{
  ...
}
...
socket.async_read_some(...,
    boost::bind(&my_class::read_handler,
      this, boost::asio::placeholders::error,
      boost::asio::placeholders::bytes_transferred));

Using use_future as a read token:

std::future<std::size_t> f =
  socket.async_read_some(..., boost::asio::use_future);
...
try
{
  std::size_t n = f.get();
  ...
}
catch (const system_error& e)
{
  ...
}

Using use_awaitable as a read token:

boost::asio::awaitable<void> my_coroutine()
{
  try
  {
    ...
    std::size_t n =
      co_await socket.async_read_some(
          ..., boost::asio::use_awaitable);
    ...
  }
  catch (const system_error& e)
  {
    ...
  }
}