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MaybeOwned<Tensor>

MaybeOwned<Tensor> is a C++ smart pointer class that dynamically encodes whether a Tensor is owned or borrowed. It is used in certain performance-sensitive situations to avoid unnecessarily incrementing a Tensor’s reference count (at a small cost in overhead from the extra indirection).

Warning

MaybeOwned must be used with extreme care. Claims of (non-)ownership are not statically checked, and mistakes can cause reference undercounting and use-after-free crashes.

Due to this lack of safety net, we discourage the use of MaybeOwned outside code paths that are known to be highly performance sensitive. However, if you encounter pre-existing uses of MaybeOwned in code that you want to modify, it’s critical to understand how to use it correctly.

The primary use case for MaybeOwned<Tensor> is a function or method that dynamically chooses between returning one of its arguments (typically from a passthrough or “no-op” code path) and returning a freshly constructed Tensor. Such a function would return a MaybeOwned<Tensor> in both cases, the former in a "borrowed" state via a call to MaybeOwned<Tensor>::borrowed(), and the latter in an "owned" state via a call to MaybeOwned<Tensor>::owned().

The canonical example is Tensor's expect_contiguous method, which shortcuts and returns a borrowed self-reference when already contiguous:

inline c10::MaybeOwned<Tensor> Tensor::expect_contiguous(MemoryFormat memory_format) const & {
  if (is_contiguous(memory_format)) {
    return c10::MaybeOwned<Tensor>::borrowed(*this);
  } else {
    return c10::MaybeOwned<Tensor>::owned(__dispatch_contiguous(memory_format));
  }
}

Using the vocabulary of lifetimes, the essential safety requirement for borrowing is that a borrowed Tensor must outlive any borrowing references to it. Here, for example, we can safely borrow *this, but the Tensor returned by __dispatch_contiguous() is freshly created, and borrowing a reference would effectively leave it ownerless.

So, general rules of thumb:

  • When in doubt, don’t use MaybeOwned<Tensor> at all - in particular, prefer avoiding using it in code that doesn’t use it already. New usage should only be introduced when critical (and demonstrable) performance gains result.
  • When modifying or calling code that already uses MaybeOwned<Tensor>, remember that it's always safe to produce a MaybeOwned<Tensor> from a Tensor in hand via a call to MaybeOwned<Tensor>::owned(). This may result in an unnecessary reference count, but never in misbehavior - so it's always the safer bet, unless the lifetime of the Tensor you're looking to wrap is crystal clear.

More details and implementation code can be found at <https://github.com/pytorch/pytorch/blob/main/c10/util/MaybeOwned.h> and <https://github.com/pytorch/pytorch/blob/main/aten/src/ATen/templates/TensorBody.h>.