### Resolve handler requirements

A resolve handler must meet the requirements for a 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 `ip::basic_resolver<>` which is used to initiate the asynchronous operation.

##### Examples

A free function as a resolve handler:

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

A resolve handler function object:

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

A lambda as a resolve handler:

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