C++11 std::list iterator invalidation after splicing

Question

I'm trying to understand how std::list iterators are affected after splice operations in C++11. Here is a small example that does not behave as I expect:

std::list<int> A({1, 2, 3});
std::list<int> B;
auto p = A.begin(), q = A.end();
auto p_copy = p, q_copy = q;

// True:
std::cout << "p == A.begin()? " << (p == A.begin()) << std::endl;
// True:
std::cout << "p_copy == A.begin()? " << (p_copy == A.begin()) << std::endl;

B.splice(B.end(), A, p, q);

// True:
std::cout << "p == B.begin()? " << (p == B.begin()) << std::endl;
// False, but I don't understand why:
std::cout << "p_copy == A.begin()? " << (p_copy == A.begin()) << std::endl;
// True, but I don't understand why either:
std::cout << "p_copy == B.begin()? " << (p_copy == B.begin()) << std::endl;

In this example, I don't understand why the p_copy iterator no longer points to A.begin() after the splice.

Answer 1

list iterators point to elements. By splicing a range of elements, you've changed which container those iterators point into. The element is no longer an element of A ; it is an element of B . Iterators are preserved across splice ing, so p and p_copy will still point to those elements.

They're just iterators into different containers now.

Answer 2

Fundamentally because this is the defined behavior of list::splice :

No iterators or references become invalidated, the iterators to moved elements remain valid, but now refer into *this , not into other.