# Making priority_queue and flat_set work with move-only types

Using a std::priority_queue with move-only elements is pretty clunky. (This has been known for a long time: Usenet, SO…)

struct T {
std::unique_ptr<int> p_;
explicit T(int i) : p_(std::make_unique<int>(i)) {}
int value() const { return *p_; }
friend auto operator<=>(const T& a, const T& b) { return a.value() <=> b.value(); }
};

std::priority_queue<T> pq;
pq.emplace(1);
pq.emplace(2);
pq.emplace(3);
assert(pq.top().value() == 3);


The trouble hits when you want to extract the top value for processing outside the queue (Godbolt):

T t = pq.pop();  // nope, returns void
T t = pq.top();  // nope, returns const T&, T isn't copyable


See, pq can’t allow you to access pq.top() as a mutable T&, because that would allow you to change its value, which might make it no longer the biggest element — breaking the priority queue’s invariant. It’s the same reason you’re not allowed to get mutable access to the elements of a sorted set (or, in C++23, flat_set).

std::set<T> s;
s.emplace(1);
s.emplace(2);
s.emplace(3);
auto it = s.begin();
assert(it->value() == 1);
T t = *it;          // nope, returns const T&, T isn't copyable
T t = s.erase(it);  // nope, returns iterator


What we need is a method that can move-from the top element and remove it from the container “in one breath,” so that the container’s invariant is preserved. This “extracting erase” operation might be considered the opposite of the “constructing insert” emplace, so the name displace seems plausible to me.

T t = pq.displace_top();  // make pq one element shorter
T t = s.displace(it);     // erase(it), returning move(*it)


I’ve just implemented this in my fork of libc++, so you can try it out on Godbolt (once the nightly build runs, anyway).

## set can kind of already do this

set and unordered_set actually do allow you to displace elements today, via the node-handle API. This is of course very silly-looking, but it has exactly the correct effect at runtime:

T t = std::move(s.extract(it).value());


However, this doesn’t work for priority_queue and flat_set, because they’re both based on an underlying sequence container (like a vector), rather than a collection of “nodes” that you can unhook individually at will.

## This doesn’t relate much to trivial relocatability

When I started writing this post, I thought relocation would be huge help here! After all, “move out of a thing, and destroy the source” is the definition of relocation. But after implementing set::displace, unordered_set::displace, flat_set::displace, and priority_queue::displace_top, I think trivial relocatability would help displace only when the container is low-level enough to manage the lifetimes of all its elements manually (without indirection). For example, vector::displace could be implemented as:

T displace(const_iterator pos) {
if (std::is_nothrow_relocatable_v<T>) {
T t = std::relocate(p);
std::uninitialized_relocate(p + 1, end_--, p);
return t;
} else {
T t = std::move(*p);
std::copy(p + 1, end_--, p);
std::destroy_at(end_);
return t;
}
}


but this much simpler implementation costs only a single extra move/destroy cycle on *p:

T displace(const_iterator pos) {
T t = std::move(*p);
erase(pos);
return t;
}


## This distantly relates to priority_queue::replace_top

When I started writing this post, I thought that perhaps priority_queue::displace could be implemented as “relocate out of the first element; relocate upward its largest child; relocate upward its largest child, etc.” in the same way that today we implement pop_heap as “move out of the first element; move-assign upward its largest child, etc.”

“Wait, I thought pop_heap started by swapping a leaf element to the top and sifting it down!” This more efficient algorithm is known as “heapsort with bounce” and was implemented in libc++ 15.

But we can’t actually do that. The elements in the priority_queue aren’t owned by the priority_queue; they’re owned by the underlying vector. So the priority_queue can’t just relocate-out-of them (that is, end their lifetimes) willy-nilly. Neither could an STL algorithm analogous to pop_heap.

There are two actually safe ways to implement priority_queue::displace_top:

// Implementation #1: swap, sift down, displace
std::pop_heap(c.begin(), c.end(), comp);
return c.displace_back();

// Implementation #2: move, displace, move-assign, destroy, sift down
value_type v = std::exchange(c.front(), c.displace_back());
std::poke_heap(c.begin(), c.end(), comp);
return v;


The poke_heap algorithm used in Implementation #2 is the same algorithm we need for priority_queue::replace_top! See priority_queue is missing an operation” (2018-04-27). Unfortunately, as you can see from the comments above, Implementation #1 gives better codegen anyway. It might help to have a relocation-based version of std::exchange, but C++ isn’t great at “functions that take recipes for prvalues as arguments.” See “Superconstructing super elider, round 2” (2018-05-17).

// Implementation #3: relocate, displace, sift down
value_type v = std::relocating_exchange(c.front(), []{ return c.displace_back(); });
std::poke_heap(c.begin(), c.end(), comp);
return v;

Posted 2023-03-01