vector<reference_wrapper> .. things going out of scope? how does it work?

Question

Use case: I am converting data from a very old program of mine to a database friendly format. There are parts where I have to do multiple passes over the old data, because in particular the keys have to first exist before I can reference them in relationships. So I thought why not put the incomplete parts in a vector of references during the first pass and return it from the working function, so I can easily use that vector to make the second pass over whatever is still incomplete. I like to avoid pointers when possible so I looked into std::reference_wrapper<T> which seemes like exactly what I need .. except I don't understand it's behavior at all.

I have both vector<OldData> old_data and vector<NewData> new_data as member of my conversion class. The converting member function essentially does:

//...
vector<reference_wrapper<NewData>> incomplete;
for(const auto& old_elem : old_data) {
    auto& new_ref = *new_data.insert(new_data.end(), convert(old_elem));
    if(is_incomplete(new_ref)) incomplete.push_back(ref(new_ref));
}
return incomplete;

However, incomplete is already broken immediately after the for loop. The program compiles, but crashes and produces gibberish. Now I don't know if I placed ref correctly, but this is only one of many tries where I tried to put it somewhere else, use push_back or emplace_back instead, etc. .. Something seems to be going out of scope, but what? both new_data and old_data are class members, incomplete also lives outside the loop, and according to the documentation, reference_wrapper is copyable.

Here's a simplified MWE that compiles, crashes, and produces gibberish:

// includes ..
using namespace std;
int main() {
    int N = 2; // works correctly for N = 1 without any other changes ... ???
    vector<string> strs;
    vector<reference_wrapper<string>> refs;
    for(int i = 0; i < N; ++i) {
        string& sref = ref(strs.emplace_back("a"));
        refs.push_back(sref);
    }
    for (const auto& r : refs) cout << r.get(); // crash & gibberish
}

This is g++ 10.2.0 with -std=c++17 if it means anything. Now I will probably just use pointers and be done, but I would like to understand what is going on here, documentation / search does not seem to help..

Answer 1

The problem here is that you are using vector data structure which might re-allocate memory for the entire vector any time that you add an element, so all previous references on that vector most probably get invalidated, you can resolve your problem by using list instead of vector.