why does it go wrong after push_back?

Question

The code here is simple:

int main() {
 vector<string> v;
 v.push_back("hello");
 string& x = v[0];
 v.push_back("world");
 cout << x << endl;
 return 0;
}

Why is there a runtime error? Please explain it in detail.

• Algirdas Preidžius: According to the documentation of std::vector::push_back (did you even look at it?): If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated. (emphasis mine)

Answer 1

Have a read through a reference : std::vector::push_back. Specifically the part that says :

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

When you pushed back the second string into the vector, it appears it had to re-allocate memory, because there wasn't enough capacity for the second string. This re-allocation moved the entire vector data elsewhere in memory - including the first string you pushed into it. So the reference to that first string became a dangling reference, and de-referencing it has undefined behavior.