### Wait handler requirements

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

##### Examples

A free function as a wait handler:

    void wait_handler(
        const asio::error_code& ec)
    {
      ...
    }

A wait handler function object:

    struct wait_handler
    {
      ...
      void operator()(
          const asio::error_code& ec)
      {
        ...
      }
      ...
    };

A lambda as a wait handler:

    socket.async_wait(...,
        [](const asio::error_code& ec)
        {
          ...
        });

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

    void my_class::wait_handler(
        const asio::error_code& ec)
    {
      ...
    }
    ...
    socket.async_wait(...,
        std::bind(&my_class::wait_handler,
          this, std::placeholders::_1));

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

    void my_class::wait_handler(
        const asio::error_code& ec)
    {
      ...
    }
    ...
    socket.async_wait(...,
        boost::bind(&my_class::wait_handler,
          this, asio::placeholders::error));