The reason of using `std::greater` for creating min heap via `priority_queue`

The logical argument is as follows

  1. std::priority_queue is a container adaptor; basic memory considerations make the back the preferred place for modifications (with pop_back() and push_back()) for sequence containers such as std::vector.
  2. the priority_queue primitives are based on std::make_heap (constructor), std::pop_heap + container::pop_back (priority_queue::pop) and on container::push_back + std::push_heap (priority_queue::push)
  3. pop_heap will take the front of the underlying storage, and put it at the back, restoring the heap invariant afterwards. The reverse goes for push_heap.
  4. doing sort_heap on a max_heap (with the max at the front initially) will repeatedly pop the front to the back and sort the range according to less (which is the default comparison operator)
  5. hence, the preferred implementation of a max_heap is to have the max element w.r.t. less at the front, accessed through priority_queue::top (underlying container::front).
  6. one can still debate whether it is intuitive that priority_queue with a std::less comparator is representing a max_heap. It could have been defined as a min_heap by reversing the comparator’s arguments (but see the comment by @T.C. that with C++98 binders this is rather verbose) everywhere in the calls to the various heap functions. The one (for me) counter-intuitive result would have been that top() would then not have given the element with top priority

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