### 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.