Why can't I push_back an object into a vector of objects that belongs to another class?

Question

What I'm trying to do here is to add Voter to a vector that belongs to RegLogBook and, RegLogBook is an object that is owned by my Election class. I don't know what I'm doing wrong here, the Voter object is not being added to the vector . The relevant codes are below, I've removed all the unnecessary parts.

Election.h

class Election
{
public:
    Election();
    ~Election();
    RegLogBook getRegLogBook();
private:
    RegLogBook* _regLogBook;
};  

Election.cpp

Election::Election()
{
    _regLogBook = new RegLogBook();
}

RegLogBook Election::getRegLogBook() {
    return *_regLogBook;
}

RegLogBook.h

class RegLogBook
{
public:
    RegLogBook();
    ~RegLogBook();
    vector<Voter*> getVoterList();

private:
    vector<Voter*> _voterList;
};

RegLogBook.cpp

vector<Voter*> RegLogBook::getVoterList() {
    return _voterList;
}

Committee.cpp register voter method

void Committee::RegVoter(RegLogBook logs) {
    Voter *newVoter = new Voter();
    logs.getVoterList().push_back(newVoter); //The voter is not added to the list
}

I call this in my main()

Election *Election2018 = new Election();
Committee com1 = new Committee();
com1.RegVoter(Election2018->getRegLogBook());

Answer 1

RegLogBook::getVoterList() returns your _voterList by value . That means it copies the whole vector and returns it. Then you add elements to that copy.

To fix this, simply change your

vector<Voter*> RegLogBook::getVoterList()

to

vector<Voter*>& RegLogBook::getVoterList()
//            ^ notice the reference part

There is another problem that I initially missed. In your Committee::RegVoter method, you take the argument by value . That means the method will be invoked with a copy of your RegLogBook . You should also change

void Committee::RegVoter(RegLogBook logs)

to

void Committee::RegVoter(RegLogBook& logs)
//      again notice the reference ^

_{Thanks to Lightness Races in Orbit for pointing that out}

Remember - to work on the original object, not a copy of it, you should pass it either by a reference or a pointer . By default you should prefer passing by reference, unless you have strong argument to use pointers

Answer 2

I believe the problem is that you return the voter list by-value, meaning that a copy of the voter list is returned:

vector<Voter*> RegLogBook::getVoterList() {
    return _voterList;
}

So this line:

logs.getVoterList().push_back(newVoter);

will modify the local copy instead of the value being stored in the object instance.

Try modifying your code to return a reference (by changing the return type to be vector<Voter*>& )

vector<Voter*>& RegLogBook::getVoterList() {
    return _voterList;
}

or, maybe better, return a pointer:

vector<Voter*>* RegLogBook::getVoterList() {
    return &_voterList;
}

I prefer the last alternative, as this makes it more obvious that the returned value can be modified.

Answer 3

From what I'm seeing your code shouldn't even compile: you're using the new keyword, but you're saving its returned value into a Committee , not into a Committee* as you should. If you are using an overloaded new operator, please post the whole code.