Why after writeln the binaryHeap become empty?

[lili]

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?