Returning an Eigen::Matrix as either 1D or 2D

[kcajf]

I've got some Eigen::Matrixes that, depending on some runtime data, either represents a (row)-vector or a 2d matrix. I would like to expose them as either 1D or 2D numpy arrays depending on this runtime variable (whether or not this is a lovely API is a separate question, but it's aiming to be compatible with some existing code).

How would I approach this? I would ideally like to be able to return both no-copy views on internal Eigen::Matrix members (i.e. rv_policy::reference_internal), as well as return by-move matrices that I construct on-the-fly.

I started down a route of copying some of the code in

nanobind/include/nanobind/eigen/dense.h

Lines 159 to 212 in 0435e16

	static handle from_cpp(const T &v, rv_policy policy, cleanup_list *cleanup) noexcept {
	size_t shape[ndim_v<T>];
	int64_t strides[ndim_v<T>];
	if constexpr (ndim_v<T> == 1) {
	shape[0] = v.size();
	strides[0] = v.innerStride();
	} else {
	shape[0] = v.rows();
	shape[1] = v.cols();
	strides[0] = v.rowStride();
	strides[1] = v.colStride();
	}
	void *ptr = (void *) v.data();
	switch (policy) {
	case rv_policy::automatic:
	policy = rv_policy::copy;
	break;
	case rv_policy::automatic_reference:
	policy = rv_policy::reference;
	break;
	case rv_policy::move:
	// Don't bother moving when the data is static or occupies <1KB
	if ((T::SizeAtCompileTime != Eigen::Dynamic ||
	(size_t) v.size() < (1024 / sizeof(Scalar))))
	policy = rv_policy::copy;
	break;
	default: // leave policy unchanged
	break;
	}
	object owner;
	if (policy == rv_policy::move) {
	T *temp = new T(std::move(v));
	owner = capsule(temp, [](void *p) noexcept { delete (T *) p; });
	ptr = temp->data();
	policy = rv_policy::reference;
	} else if (policy == rv_policy::reference_internal && cleanup->self()) {
	owner = borrow(cleanup->self());
	policy = rv_policy::reference;
	}
	object o = steal(NDArrayCaster::from_cpp(
	NDArray(ptr, ndim_v<T>, shape, owner, strides),
	policy, cleanup));
	return o.release();
	}
	};

, but I seemed to end up needing to reimplement all of the ownership management stuff, and started to have to put rv_policy values inside my functions rather than just on the bindings. I wonder if there is a simpler way to approach this?

Thanks!