Why do I get a compile error if I don't include an empty constructor for this class?

Question

I have a Client class which throws a compile error:

class Client {
public:
    struct sockaddr_in* sa;
    int roomid;
    Client(struct sockaddr_in* socka, int id=0) : sa(socka), roomid(id) {}
};

Error:

chatserver.cc:37:2: note:   candidate expects 2 arguments, 0 provided
chatserver.cc:33:7: note: candidate: ‘constexpr Client::Client(const Client&)’
   33 | class Client {
      |       ^~~~~~

However, if I add an empty constructor, Client() {}, then it compiles fine.

Why?

EDIT:

I've condensed the code so it's more readable.

Basically, I initialise an unordered_map, then at the beginning of a while loop, I loop through the map to print its contents. Further downstream the loop, I listen on a UDP socket and add a new Client to the map:

  unordered_map<string, Client> ip_client_map;

  while (true) {
      for (auto const& x : ip_client_map) {
          cout << x.first << '=' << x.second.sa->sin_addr.s_addr << ':' << ntohs(x.second.sa->sin_port) << endl;
      }
      struct sockaddr_in src;
      socklen_t srclen = sizeof(src);
      char buf[1000];
      int rlen = recvfrom(sockfd, buf, sizeof(buf)-1, 0, (struct sockaddr*) &src, &srclen);
      buf[rlen] = 0;

      string srcaddr = string(inet_ntoa(src.sin_addr)) + ":" + to_string(ntohs(src.sin_port));

      struct sockaddr_in* sa = (sockaddr_in*) malloc(sizeof(struct sockaddr_in));
      memcpy(sa, &src, sizeof(struct sockaddr_in));
      Client new_client = Client(sa);
      ip_client_map[srcaddr] = new_client;

  }

Sidenote: each time I print the contents of the unordered_map at the beginning of the while loop, the IP address and port number are 0 and 0, so I think I'm not copying the sockaddr_in* properly.

See the dupe, you're calling the default constructor somewhere in your code (which you haven't shown). — cigien, Nov 10 '20 at 11:29
Seems like you’re creating a Client object using the default constructor somewhere. — splrs, Nov 10 '20 at 11:29
I added in the code. Not using the default constructor anywhere for sure, but I'm wondernig if it has to do with iterating through a freshly initialised map — doctopus, Nov 10 '20 at 11:39
Yes, it's the map. `std::map::operator[]` will create a new element if the key doesn't exist. And guess what constructor it will (try to) use ;) — Lukas-T, Nov 10 '20 at 11:58
When you use `operator[]` of `std::unordered_map`, then you inserts value that is value initialized. You need to use `emplace()` or `insert()`. — sklott, Nov 10 '20 at 11:58

Why do I get a compile error if I don't include an empty constructor for this class?

0 Answers0