stl · high

Iterator Invalidation

Why an iterator into a vector dangles after a reallocation, and how reserve / indices / erase's return value avoid it.

The code

std::vector<int> v = {10, 20, 30, 40};
auto it = v.begin() + 1; // points at 20
v.push_back(50);         // may REALLOCATE -> it dangles!
// *it;                      // undefined behavior if reallocated

v.reserve(8);     // fix A: pre-allocate, no realloc
size_t i = 1;     // fix B: an index survives realloc
it = v.erase(it); // erase returns the next valid iterator

What this lesson walks through

1. 01An iterator points INTO the buffer
2. 02push_back full -> reallocation moves the buffer
3. 03it now dangles -> UB
4. 04Fix A: reserve() up front
5. 05Fix B: use an index, not an iterator
6. 06erase invalidates from that point on
7. 07The invalidation rules

it = v.begin() + 1 holds a raw address inside the vector's buffer - it points directly at the element 20.