Question

I’m trying to add some songs to a vector inside a class. One of the values I’m storing is an int representing the song. It’s essentially a counter. The first song I add should have the value 1, the second value two and so forth. But It’s getting other strange values like big random numbers (positives and negatives). I can’t wrap my head around what I’m doing wrong. This is the code:

#include <iostream>
#include <vector>
#include <string>

using namespace std;

class Jukebox{
public:
  void addSong(string artist, string title, string filename) {
    song s {++songCounter, artist, title, filename};
    Songs.push_back(s);
  }

  void printSong (int song) {
    cout << Songs[song].no << ". ";
    cout << Songs[song].artist << " - ";
    cout << Songs[song].title << " : ";
    cout << Songs[song].filename << endl;
  }

private:
  struct song {
    int no;
    string artist;
    string title;
    string filename;
  };
  vector<song> Songs;
  int songCounter;
};

int main() {
  Jukebox jbox;
  jbox.addSong("U2", "Magnificent", "U2-Magnificent.mp3");
  jbox.addSong("Sting", "Englishman in New York", "Sting-Englishman_in_New_York.mp3");
  jbox.addSong("U2", "One", "U2-One.mp3");
  jbox.printSong(0);
  jbox.printSong(1);
  jbox.printSong(2);
  return 0;
}

Update

Ok, I’m probably stupid and should read more about classes before trying to implement this. But I think I did read and I still don’t get it. This is what my class looks like now (which won’t work):

class Jukebox(): songCounter(0)
 {

public:
  void addSong(string artist, string title, string filename) {
    songCounter++;
    song s {songCounter, artist, title, filename};
    Songs.push_back(s);
  }

  void printSong (int song) {
    cout << Songs[song].no << ". ";
    cout << Songs[song].artist << " - ";
    cout << Songs[song].title << " : ";
    cout << Songs[song].filename << endl;
  }

private:
  int songCounter;
  struct song {
    int no;
    string artist;
    string title;
    string filename;
  };
  vector<song> Songs;
};

Final word

Ok. From the example I’ve seen of c++ contructor classes I had some kind of wrong impression of how they worked. Now I think I’m getting it a little bit more. But the syntax still seems strange to me. But I try to read more so I really understand it. Here is what I did and to seems to work:

#include <iostream>
#include <vector>
#include <string>

using namespace std;

class Jukebox {

public:
  void addSong(string artist, string title, string filename) {
    songCounter++;
    song s {songCounter, artist, title, filename};
    Songs.push_back(s);
  }

  void printSong (int song) {
    cout << Songs[song].no << ". ";
    cout << Songs[song].artist << " - ";
    cout << Songs[song].title << " : ";
    cout << Songs[song].filename << endl;
  }

  Jukebox(): songCounter(0) {} // Constructor

private:
  int songCounter;
  struct song {
    int no;
    string artist;
    string title;
    string filename;
  };
  vector<song> Songs;
};

int main() {
  Jukebox jbox;
  jbox.addSong("U2", "Magnificent", "U2-Magnificent.mp3");
  jbox.addSong("Sting", "Englishman in New York", "Sting-Englishman_in_New_York.mp3");
  jbox.addSong("U2", "One", "U2-One.mp3");
  jbox.printSong(0);
  jbox.printSong(1);
  jbox.printSong(2);
  return 0;
}

Answer 1

You did not initialize songCounter in your constructor.

Jukebox(): songCounter(0),//....other members

If you do not initialize it, then it may have any random value and that leaves your program in an Undefined State.

Always be careful while using unitialized variables, it often leads to Undefined Behavior and your program is a good example of it.

Also, I am not sure of your design but probably it should be a static member if you want to use it as a counter, which maintains state for all objects of your Song class.
Or
You will have to explicitly set it to a proper value at time of creating a Song object.

Okay its a counter for JukeBox and not Song class so its still okay to be a member.