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### Write handler requirements

A write handler must meet the requirements for a handler. A value `h` of a write 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 write handler:

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

A write handler function object:

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

A lambda as a write handler:

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

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

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

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

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