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### Read handler requirements

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 boost::system::error_code& ec,
        std::size_t bytes_transferred)
    {
      ...
    }

A read handler function object:

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

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

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