Reference types with metadata cause problems

Vinnie Falco, in a draft of an upcoming paper P1100, writes:

A dynamic string buffer contains a reference to the underlying string. Copies of a dynamic string buffer refer to the same string. Note that the dynamic string buffer also contains some state: the size_ and max_size_ data members. This additional metadata informs the dynamic string buffer of the boundaries between the readable and writable bytes, as well as the maximum allowed size of the total of the readable and writable bytes.

Vinnie quotes LWG 3072, filed by Chris Kohlhoff:

Asio … performs DECAY_COPY(b) in the async_read, async_write, and async_read_until initiating functions. All operations performed on b are actually performed on that decay-copy, or on a move-constructed descendant of it. The copy is intended to refer to the same underlying storage and be otherwise interchangeable with the original in every way. [Ed: emphasis added]

The core of the problem is:

When only one composed operation handles the dynamic buffer, things seem to work. However, if a composed operation wishes to invoke another composed operation on that dynamic buffer, […] it cannot do so without depending on undefined behavior.

We saw the same problem come up (at least teachability-wise) with P0059R4 ring_span, on which I am a coauthor. A ring_span contains a reference to the underlying buffer, and also contains some state: the m_size and m_front_idx data members. This additional metadata informs the ring_span object of the boundaries between the elements which are currently “inside the ring-buffer” and those which are currently “unused capacity.”

Like Vinnie, I found that this was a confusing design for a C++ class type. A ring_span looks like a non-owning reference type, so the temptation is very strong to write e.g.

void consume_from(ring_span<Task> r) {
    Task t = r.pop_front();

void produce_to(ring_span<Task> r) {
    Task t = newtask();

passing the ring_span object by value the same way you’d pass a string_view or a function_ref. However, if you do this, you make a copy of the size_ and front_idx_ metadata members, and all your side-effects act on that copy, and the caller never sees them! Even if your program doesn’t crash as a result, you’ve still got two ring_span objects who both think they know what’s going on with the buffer, and at least one of them is wrong.

“Data structure views”

The other day I had a discussion with Charley Bay, Niall Douglas, and Bob Steagall on the subject of what at-least-some-of-them wanted to call “addressing engine” types. The idea, essentially, is that we can think of a ring as simply a ring_span slapped on top of a std::vector (or indeed a std::array) — and how can we generalize this idea?

The Standard already provides three types kinda-sorta of this nature: stack, queue, and priority_queue. There are two big differences between std::priority_queue and P0059 ring_span:

  • priority_queue owns its elements. The nature of its owning container is template-parameterized, but it definitely is owning.

  • Imagine for a minute that priority_queue was not owning. Then, still, priority_queue holds no metadata — if you know the number and arrangement of the elements in its storage container, you have everything you need to reconstruct a new priority_queue which would be an exact copy of the original. Contrariwise, it is impossible to reconstruct a ring_span given only the arrangement of its storage container; you also need to know which element to consider the “front” of the ring-buffer.

Come to think of it, this boils down to just one difference: priority_queue is backed by a resizable, owning STL container, whereas ring_span is backed by a raw range. If we designed a priority_queue backed by a raw range, then either it would not be resizable (which is actually a perfectly cromulent design for a priority queue!) or else it would have to keep track of its “current size” (size_) as a data member — at which point, passing it around by value would be just as dangerous as passing around a ring_span or a dynamic_string_buffer by value.

Notice that when you pass a string_view around by value, you don’t have these problems, even though string_view also contains a “size” data member and has operations such as sv.remove_prefix(k) which change the “size.” The reason, I think, is that when you change the size of a string_view, you intuitively understand that you are not destroying the unviewed elements. Whereas, when you “pop” an element off of a priority_queue, you understand that the popped element is really gone, not merely unviewed.

P0059’s ring_span tries to split the difference. Its mental model is that unviewed elements are still there, just like string_viewbut, whenever an element transitions from the “viewed” state to the “unviewed” state, we run a special bit of code (customizable via the template parameters) called the “popper,” which defaults to leaving the popped element in a moved-from state.

I have come to the position that the only really foolproof way to use P0059 ring_span is to wrap it up in a container adaptor, something like this:

template<class T, class Container = std::vector<T>>
class ring {
    Container c;
    ring_span<T> rs;
    explicit ring(size_t capacity) :
        c(capacity), rs(c.begin(), c.end()) {}
    void push_back(T t) { rs.push_back(t); }
    T pop_front() { return rs.pop_front(); }
    size_t size() const { return rs.size(); }

This doesn’t mean that ring_span is useless on its own; actually I think it might be quite useful in some (experts-only) cases.

Which raises the question: Does it make sense to have foo_span types for other containers too? Could we imagine a vector_span that doesn’t own its memory, doesn’t know anything about allocators, but does track its own size_ distinct from its maximum capacity?

I believe that we could imagine such a vector_span, but it would have all the problems that ring_span has: it tempts you to inappropriately pass-by-value, and it raises troubling questions about the lifetimes of “unviewed” objects (which even ring_span’s “popper” solution doesn’t fix as much as I’d like to). And unlike ring_span, it seems not to abstract away anything useful from the owning version of the container. Consider:

template<class T, class Container = std::vector<T>>
class vector {
    Container c;
    vector_span<T> vs;
    explicit ring(size_t capacity) :
        c(capacity), vs(c.begin(), c.end()) {}
    void push_back(T t) { vs.push_back(t); }
    T pop_front() { return vs.pop_front(); }
    size_t size() const { return vs.size(); }

Here we have combined a vector_span with an owning STL container to produce an owning vector. Unfortunately, the only sensible choice for the underlying STL container is… vector itself, leading to a silly circular definition. Why does this example seem silly? Is vector just too simple to be a suitable testbed for our “non-owning non-allocating view” idea?

Taking it even further, could we imagine a non-owning, non-allocating type analogous to std::set (that is, a red-black tree) or std::list? I have tried, but I can’t imagine what such a thing would look like.

So we seem to have a continuum here, with vector at the near end, ring in the middle, and set at the far end. At the near end, the idea of a “non-owning, non-allocating view” is quickly reduced ad absurdum; at the far end, the idea is unimaginable (at least to me); and there’s a sweet spot in the middle, exemplified by ring_span and dynamic_string_buffer, where the idea seems workable but has significant teachability and usability problems.

Disclaimer: I may be too close to ring_span, and too relatively far from vector_span and set_span, to really judge how “workable” any of them are. Maybe you can implement a set_span and show me what it looks like; or maybe you consider ring_span’s flaws to be fatal (meaning that the spot in the middle is no sweeter than anywhere else on the continuum).

Anyway, the take-home point here is: when you design a type that represents a non-owning reference to some objects, plus some other data members, you are signing up for a whole lot of trouble. I recommend against it.

Posted 2018-05-30