DNS query

The resolver class is used to do DNS query, i.e., convert a host+service to IP+port. Take boost::asio::ip::tcp::resolver as an example: 

  1. #include <boost/asio.hpp>
  2. #include <iostream>
  4. int main()
  5. {
  6.     try
  7.     {
  8.         boost::asio::io_context io_context;
  10.         boost::asio::ip::tcp::resolver resolver{io_context};
  11.         boost::asio::ip::tcp::resolver::results_type endpoints =
  12.                 resolver.resolve("google.com", "https");
  14.         for (auto it = endpoints.cbegin(); it != endpoints.cend(); it++)
  15.         {
  16.             boost::asio::ip::tcp::endpoint endpoint = *it;
  17.             std::cout << endpoint << '\n';
  18.         }
  19.     }
  20.     catch (std::exception& e)
  21.     {
  22.         std::cerr << e.what() << '\n';
  23.     }
  25.     return 0;
  26. }

The running result is: 

  1. 74.125.24.101:443
  2. 74.125.24.139:443
  3. 74.125.24.138:443
  4. 74.125.24.102:443
  5. 74.125.24.100:443
  6. 74.125.24.113:443

The element of boost::asio::ip::tcp::resolver::results_type‘s every iteration is basic_resolver_entry: 

  1. template <typename InternetProtocol>
  2. class basic_resolver_entry
  3. {
  4. ......
  5. public:
  6.   /// The protocol type associated with the endpoint entry.
  7.   typedef InternetProtocol protocol_type;
  9.   /// The endpoint type associated with the endpoint entry.
  10.   typedef typename InternetProtocol::endpoint endpoint_type;
  11. ......
  12.   /// Convert to the endpoint associated with the entry.
  13.   operator endpoint_type() const
  14.   {
  15.     return endpoint_;
  16.   }
  17. ......
  18. }

Since it has endpoint_type() operator, it can be converted to endpoint directly: 

  1. boost::asio::ip::tcp::endpoint endpoint = *it;