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:

.. code-block:: cpp

  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/master/c10/util/MaybeOwned.h> and
<https://github.com/pytorch/pytorch/blob/master/aten/src/ATen/templates/TensorBody.h>.
