C++ std::vector Iterator/Pointer Invalidation


Iterators and pointers pointing into an std::vector can become invalid, but only when performing certain operations. Using invalid iterators/pointers will result in undefined behavior.

Operations which invalidate iterators/pointers include:

  • Any insertion operation which changes the capacity of the vector will invalidate all iterators/pointers:

    vector<int> v(5); // Vector has a size of 5; capacity is unknown.
    int *p1 = &v[0];
    v.push_back(2);   // p1 may have been invalidated, since the capacity was unknown.
    v.reserve(20);    // Capacity is now at least 20.
    int *p2 = &v[0];
    v.push_back(4);   // p2 is *not* invalidated, since the size of `v` is now 7.
    v.insert(v.end(), 30, 9); // Inserts 30 elements at the end. The size exceeds the
                              // requested capacity of 20, so `p2` is (probably) invalidated.
    int *p3 = &v[0];
    v.reserve(v.capacity() + 20); // Capacity exceeded, thus `p3` is invalid.
auto old_cap = v.capacity();
if(old_cap != v.capacity())
    // Iterators were invalidated.
  • Any insertion operation, which does not increase the capacity, will still invalidate iterators/pointers pointing to elements at the insertion position and past it. This includes the end iterator:

    vector<int> v(5);
    v.reserve(20);                 // Capacity is at least 20.
    int *p1 = &v[0];
    int *p2 = &v[3];
    v.insert(v.begin() + 2, 5, 0); // `p2` is invalidated, but since the capacity
                                   // did not change, `p1` remains valid.
    int *p3 = &v[v.size() - 1];
    v.push_back(10); // The capacity did not change, so `p3` and `p1` remain valid.
  • Any removal operation will invalidate iterators/pointers pointing to the removed elements and to any elements past the removed elements. This includes the end iterator:

    vector<int> v(10);
    int *p1 = &v[0];
    int *p2 = &v[5];
    v.erase(v.begin() + 3, v.end()); // `p2` is invalid, but `p1` remains valid.
  • operator= (copy, move, or otherwise) and clear() will invalidate all iterators/pointers pointing into the vector.