C++ base vector refer to derived vector

Question

Why is this code not working? There are two vectors. The base vector elements should refer to the derived vector elements as a reference (not a copy). The output is random numbers, so, for some reason, the reference_wrapper elements doesn't refer correctly to the derived elements but.

#include <iostream>
#include <vector>

struct Base
{
    Base (int a) : a_ (a) {}
    int a_;
};

struct DerivedA : Base
{
    DerivedA (int a) : Base (a) {}
};

int main(int argc, const char * argv[]) {
    std::vector <std::reference_wrapper <Base>> b;
    std::vector <DerivedA> d;

    const int num = 100;
    for (int i = 0; i < num; i++)
    {
        d.push_back (i);
        b.push_back (d.back());
    }

    for (int i = 0; i < num; i++)
        std::cout << b[i].get().a_;

    return 0;
}

`push_back` can invalidate all references to the existing elements. — François Andrieux, Jul 16 '18 at 13:28
Have a look through https://stackoverflow.com/questions/6438086/iterator-invalidation-rules — NathanOliver, Jul 16 '18 at 13:29
If you really want to do things this way, use a `std::deque` instead. Unlike `std::vector`, `std::deque` won't move its elements around when you push elements to the back, so those references will still be valid. — Bernard, Jul 16 '18 at 16:51

score 7 · Answer 1 · answered Jul 16 '18 at 13:32

If you look into documentation of std::vector::push_back() it clearly says:

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated.

Emphasis is mine. So this happens in your case, to fix it you need to make sure that capacity of your vector is big enough or use another container, which does not invalidate. For example std::deque

All iterators, including the past-the-end iterator, are invalidated. No references are invalidated.

Or store indexes of elements instead of references.

Or push all the values into `d`, then loop again and push the reference wrappers into `b`. — Pete Becker, Jul 16 '18 at 14:47

C++ base vector refer to derived vector

1 Answers1