Re: Broken interaction between std::priority_queue and move-only types

Wil Evers <bouncer@dev.null>
Mon, 21 May 2012 14:24:18 -0700 (PDT)
Seungbeom Kim wrote:

On 2012-05-20 21:19, Zoltan Juhasz wrote:


for example:

value_type faulty_pop()
  auto e = move_if_noexcept( internal_top() );
  return e;

does not provide strong exception guarantees, b/c when you return
'e', an exception might occur, and at that point you've changed the
container in an irreversible way.

Can't we enable that function only for value_types with a
non-throwing move constructor?

The pop( value_type& e ), on the other hand, does not suffer from
these kind of problems;

But then you need to default construct an object of value_type and
ask the top element to be move-assigned to it. It may not be always
possible or desirable to default construct such an object, just to
be overwritten later.

I agree. Paradoxically, a movable type without a default constructor
seems like a strange beast, because it presumably implements a "valid,
but resourceless" state for its objects to be left in after having
been moved from.

I would say that a type with a default constructor clearly advertises
that such a state exists and must be reckoned with.

- Wil

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