vector push_back not working in class object - c++

Question

I am making an hierarchy of sorts and am having trouble adding an element to a vector. I simplified the code and still cannot add an element to the vector as expected. The hierarchy looks like:
Pdb > Chain > strings
Pdb and Chain are class names and strings is the name of a vector belonging to Chain. I cannot push_back to add a string to strings. You may get a better idea looking at my code:
Chain Class:
class Chain {
string chain_id;
vector<string> strings;
public:
Chain(string id_) { chain_id = id_; }
vector<string> GetStrings() { return strings; }
void AddString(string s) {
cout << "Size of strings BEFORE push_back in AddString: " << strings.size() << endl;
strings.push_back(s);
cout << "Size of strings AFTER push_back in AddString: " << strings.size() << endl;
}
string GetChainId() { return chain_id; }
};
Pdb class:
class Pdb {
string pdb_id;
vector<Chain> chains;
public:
Pdb(string id_) { pdb_id = id_; }
vector<Chain> GetChains() { return chains; }
void AddChain(Chain c) { chains.push_back(c); }
string GetPdbId() { return pdb_id; }
};
main:
int main () {
vector<Pdb> pdbs;
pdbs.push_back(Pdb("1ubq"));
cout << "\n\t1. " << pdbs[0].GetPdbId() << endl;
pdbs[0].AddChain(Chain("A"));
cout << "\n\t2. " << pdbs[0].GetChains()[0].GetChainId() << endl;
pdbs[0].GetChains()[0].AddString("Whateva");
cout << "\n\t3. Size of strings after AddString in main: " << pdbs[0].GetChains()[0].GetStrings().size() << endl;
return 0;
}
This outputs:
1. 1ubq
2. A
Size of strings BEFORE push_back in AddString: 0
Size of strings AFTER push_back in AddString: 1
3. Size of strings after AddString in main: 0
As you can see the AddString function is adding an element to stings within the AddString function itself but when I GetStrings back in main, after executing AddString, strings is empty. I do not understand why this is happening. Any help would be much appreciated.

Answer 1

The problem is that you return a copy of the member, not the member itself:
vector<Chain> GetChains()
should be
vector<Chain>& GetChains()
for this to work.
I must note that you're heavily breaking the single responsibility principle. You're operating on members directly, which can't be a good idea. Consider replacing:
pdbs[0].GetChains()[0].AddString("Whateva");
with something like
pdbs[0].AddStringToChain(0,"Whateva");