I know that there are a lot of questions somewhat related to this one, but they answers are a bit hard for me to make sense of. I'm receiving the following error for a few different lines of code: C:\Users\Jeff\AppData\Local\Temp\ccAixtmT.o:football.cpp:(.text+0x6f0): undefined reference to `Player::set_values(int, std::string, float)' From these blocks of code: class Player { int playerNum; string playerPos; float playerRank; public: void set_values(int, string, float); float get_rank(){ return playerRank; }; bool operator == (const Player &p1/*, const Player &p2*/) const { if(&p1.playerNum == &playerNum && &p1.playerPos == &playerPos && &p1.playerRank == &playerRank) return true; else return false; }; }; And this being the main function referencing the subclass: int main() { ifstream infile; infile.open ("input.txt", ifstream::in); int numTeams; string command; while(!infile.fail() && !infile.eof()){ infile >> numTeams; string name; Player p; int playNum; string playPos; float playRank; Player all[11]; float ranks[11]; Team allTeams[numTeams]; for(int i=0; i<numTeams; i++){ infile >> name; for(int j=0; j<11; j++){ infile >> playNum; infile >> playPos; infile >> playRank; if(playPos == "QB") p.set_values(playNum, playPos, (playRank*2.0)); else if(playPos == "RB") p.set_values(playNum, playPos, (playRank*1.5)); else if(playPos == "WR") p.set_values(playNum, playPos, (playRank/1.8)); else if(playPos == "TE") p.set_values(playNum, playPos, (playRank*1.1)); else if(playPos == "GD") p.set_values(playNum, playPos, (playRank/2.0)); else if(playPos == "TC") p.set_values(playNum, playPos, (playRank/2.2)); else if(playPos == "CR") p.set_values(playNum, playPos, (playRank/1.2)); all[j] = p; allTeams[i].set_values(all, name); } } infile >> command; if (command == "play"){ int t1; int t2; infile >> t1; infile >> t2; play(allTeams[t1], allTeams[t2]); } else { int t1; int p1; int t2; int p2; swap(allTeams[t1], allTeams[t1].get_player(p1), allTeams[t2], allTeams[t2].get_player(p2)); } } }