C++11 forward_list iterator remains pointing to removed value

Question

An iterator remains to point to the element it previously did and the value it points to is not removed from memory. I wonder how can this be explained? Thank you.

forward_list<int> Fwdl {46, 21, 88, 901, 404};
auto it = Fwdl.begin();

Fwdl.remove(46);

cout << "List : "; for(int& elem : Fwdl) cout << " " << elem; cout << endl;

//This prints "List : 21 88 901 404" as expected, but:

cout << "Iterator is alive! " << *it << endl;

//This still prints "Iterator is alive! 46"

Answer 1

N4431 - 23.3.5.5/15 list operations [list.ops] (emphasize mine)

void remove(const T& value);
template <class Predicate> void remove_if(Predicate pred);

Effects: Erases all the elements in the list referred by a list iterator i for which the following conditions hold: *i == value, pred(*i) != false. Invalidates only the iterators and references to the erased elements.

What you have is a typical manifestation of undefined behaviour and you should not rely on such code.

What probably happens is something similar to this:

int* p = new int(42);
int* iterator = p;
delete p;

// may still display 42, since the memory may have not yet been reclaimed by the OS, 
// but it is Undefined Behaviour
std::cout << *iterator;