push_back element to vector in class object isn't working

Question

1. classObject and vector within it.
2. I made a vector of classObject and function that searches classObject with a given int id.
3. Wanted to search certain classObject from listClass and add "apple" inside its wordList but failed. (assume listClass contains classObject with id 1)

class classObject { 
 public: 
    int id;
    vector<string> wordList; 
};

vector<classObject> listClass; 

classObject getClassById(int _i) {
    for(auto e : listClass) {
        if(e.id == _i) return e;
    }
} 

classObject example = getClassById(1);

example.wordList.push_back("apple");

cout << getClassById(1).wordList.size();
// returns 0. Supposed to be 1 (added apple)

Tried to use pointer

classObject* getClassById(int _i) {
    for(auto e : listClass) {
        if(e.id == _i) return &e;
    }
} 

classObject example = *getClassById(1);

example.wordList.push_back("apple");

Still not working. help!

Answer 1

classObject example is a copy, you need to use classObject& example to store a reference and change getClassById to return a reference too:

classObject& getClassById(int _i) {
  for(auto& e : listClass) {
    if(e.id == _i) return e;
  }
} 

classObject& example = getClassById(1);

Now we're left with the problem that getClassById doesn't always return a value. You probably have three options to solve this:

1. Throw an exception at the end of getClassById when no value is found
2. Return some default object
3. Change the return from a reference to a pointer and return a null pointer

For either of the last two options you'll need to change the calling code to check for the special/null value.