Utilities

C10 provides utility classes for memory management and other common patterns.

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.

Use Case:

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 MaybeOwned<Tensor>::borrowed(), and the latter in an "owned" state via MaybeOwned<Tensor>::owned().

Example - expect_contiguous:

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

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. In the example above, we can safely borrow *this, but the Tensor returned by __dispatch_contiguous() is freshly created, and borrowing a reference would effectively leave it ownerless.

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 MaybeOwned.h and TensorBody.h.

Error Handling and Assertions

PyTorch provides macros for error checking and assertions that produce informative error messages with source location. These are defined in c10/util/Exception.h.

TORCH_CHECK

The primary macro for validating user input and runtime conditions. On failure, raises c10::Error (which becomes RuntimeError in Python).

cpp

#include <c10/util/Exception.h>

// Basic check
TORCH_CHECK(tensor.dim() == 2, "Expected 2D tensor, got ", tensor.dim(), "D");

// Without message (default message generated)
TORCH_CHECK(x >= 0);

Typed variants raise specific Python exception types:

TORCH_CHECK_INDEX(cond, ...) — raises IndexError
TORCH_CHECK_VALUE(cond, ...) — raises ValueError
TORCH_CHECK_TYPE(cond, ...) — raises TypeError
TORCH_CHECK_LINALG(cond, ...) — raises LinAlgError
TORCH_CHECK_NOT_IMPLEMENTED(cond, ...) — raises NotImplementedError

TORCH_INTERNAL_ASSERT

For internal invariants that should always hold (i.e., failures indicate a bug in PyTorch, not user error). Produces a message asking users to report the bug.

cpp

TORCH_INTERNAL_ASSERT(googol > 0);
TORCH_INTERNAL_ASSERT(googol > 0, "googol was ", googol);

{note}


Use `TORCH_CHECK` for conditions that can fail due to user input.
Use `TORCH_INTERNAL_ASSERT` only for conditions that indicate a PyTorch bug.
`TORCH_INTERNAL_ASSERT_DEBUG_ONLY` is the debug-build-only variant for
hot paths.

TORCH_WARN

Issues a warning (not an error) to the user.

cpp

TORCH_WARN("This operation is slow for sparse tensors");
TORCH_WARN_ONCE("This warning appears only once");

c10::Error

The base exception class for PyTorch C++ errors. Provides source location and optional backtrace.

cpp

try {
    auto result = some_operation();
} catch (const c10::Error& e) {
    std::cerr << e.what() << std::endl;
    // Or without backtrace:
    std::cerr << e.what_without_backtrace() << std::endl;
}

Specialized subclasses: c10::IndexError, c10::ValueError, c10::TypeError, c10::NotImplementedError, c10::LinAlgError, c10::OutOfMemoryError.