Why after writeln the binaryHeap become empty?

[lili via Digitalmars-d-learn]

Hi Guys:
   see this code
~~~
    int[] ar = [1,2,3,4,52,34,22];
    auto h = heapify(ar);
    assert(h.length() == ar.length);
    writeln("h:",h);
    assert(h.empty());
~~~
dmd v2.086.0  run all assert passed. Why?