[/
 / Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:ResolveHandler Resolve handler requirements]

A resolve handler must meet the requirements for a [link boost_asio.reference.Handler
handler]. A value `h` of a resolve handler class should work correctly in the
expression `h(ec, r)`, where `ec` is an lvalue of type `const error_code` and
`r` is an lvalue of type `const ip::basic_resolver_results<InternetProtocol>`.
`InternetProtocol` is the template parameter of the [link
boost_asio.reference.ip__basic_resolver `ip::basic_resolver<>`] which is used to
initiate the asynchronous operation.

[heading Examples]

A free function as a resolve handler:

  void resolve_handler(
      const boost::system::error_code& ec,
      boost::asio::ip::tcp::resolver::results_type results)
  {
    ...
  }

A resolve handler function object:

  struct resolve_handler
  {
    ...
    void operator()(
        const boost::system::error_code& ec,
        boost::asio::ip::tcp::resolver::results_type results)
    {
      ...
    }
    ...
  };

A lambda as a resolve handler:

  resolver.async_resolve(...,
      [](const boost::system::error_code& ec,
        boost::asio::ip::tcp::resolver::results_type results)
      {
        ...
      });

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

  void my_class::resolve_handler(
      const boost::system::error_code& ec,
      boost::asio::ip::tcp::resolver::results_type results)
  {
    ...
  }
  ...
  resolver.async_resolve(...,
      std::bind(&my_class::resolve_handler,
        this, std::placeholders::_1,
        std::placeholders::_2));

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

  void my_class::resolve_handler(
      const boost::system::error_code& ec,
      boost::asio::ip::tcp::resolver::results_type results)
  {
    ...
  }
  ...
  resolver.async_resolve(...,
      boost::bind(&my_class::resolve_handler,
        this, boost::asio::placeholders::error,
        boost::asio::placeholders::results));

[endsect]