What is iterator invalidation in C++?

A change to a container can make old iterators, pointers and references invalid. Using them is undefined behavior. For a vector, push_back can move all the items to a bigger memory block when the capacity is full.

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {1, 2, 3};
    const int* before = v.data();
    v.reserve(1000);                          // the data moves to a new place
    const int* after = v.data();
    std::cout << "data moved: " << (before != after) << '\n';
    // an iterator or pointer taken before reserve() must not be used now
}

Container	After insert	After erase
vector	All invalid if it reallocates, else from the insert point	From the erased item onward
list, map, set	Stay valid	Only the erased item is invalid
unordered_map	Can all become invalid on a rehash	Only the erased item

Safe habits: use indexes, call reserve early, and do not keep iterators across changes. See Vectors.

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