Why does std::distance return zero?

In the code below I try to calculate the distance between two iterators. The iterators start at the same position and I advance once three times, so I feel like the distance between the two should be three, but I always get 0. Why?

using namespace std;

stringstream strstream{"Hello world"};
            
auto b = istream_iterator<char>{strstream};
auto e = istream_iterator<char>{strstream};

++e;
++e;
++e;
            
cout << "distance: " << distance(b,e) << endl;

>Solution :

This is because input iterators only guarantee a single pass. The following is from cppreference about the operator++ of an input iterator:

++r

[…]

Postcondition: Any copies of the previous value of r are no longer required to be either dereferenceable or to be in the domain of ==.

So, after ++e, b no longer has any guarantee on its value or if it is even usable.