A read handler must meet the requirements for a handler. A value h of a read handler class should work correctly in the expression h(ec, s), where ec is an lvalue of type const error_code and s is an lvalue of type const size_t.

Examples

A free function as a read handler:

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

A read handler function object:

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

A lambda as a read handler:

socket.async_read(...
    [](const asio::error_code& ec,
      std::size_t bytes_transferred)
    {
      ...
    });

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

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

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

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