Separate const of ndarray from const of its data.

docs/api_extra.rst

@@ -664,6 +664,12 @@ section <ndarrays>`.
 
 .. cpp:class:: template <typename... Args> ndarray
 
+   .. cpp:var:: ReadOnly
+
+      A constant static boolean that is true if the array's data is read-only.
+      This is determined by the class template arguments, not by any dynamic
+      properties of the referenced array.
+
    .. cpp:function:: ndarray() = default
 
       Create an invalid array.
@@ -793,14 +799,19 @@ section <ndarrays>`.
       In a multi-device/GPU setup, this function returns the ID of the device
       storing the array.
 
-   .. cpp:function:: const Scalar * data() const
+   .. cpp:function:: Scalar * data() const
+
+      Return a pointer to the array data.
+      If :cpp:var:`ReadOnly` is true, a pointer-to-const is returned.
 
-      Return a const pointer to the array data.
+   .. cpp:function:: template <typename... Ts> auto& operator()(Ts... indices)
 
-   .. cpp:function:: Scalar * data()
+      Return a reference to the element stored at the provided index/indices.
+      If :cpp:var:`ReadOnly` is true, a reference-to-const is returned.
+      Note that ``sizeof(Ts)`` must match :cpp:func:`ndim()`.
 
-      Return a mutable pointer to the array data. Only enabled when `Scalar` is
-      not itself ``const``.
+      This accessor is only available when the scalar type and array dimension
+      were specified as template parameters.
 
    .. cpp:function:: template <typename... Extra> auto view()
 
@@ -813,14 +824,6 @@ section <ndarrays>`.
       ``shape()``, ``stride()``, and ``operator()`` following the conventions
       of the `ndarray` type.
 
-   .. cpp:function:: template <typename... Ts> auto& operator()(Ts... indices)
-
-      Return a mutable reference to the element at stored at the provided
-      index/indices. ``sizeof(Ts)`` must match :cpp:func:`ndim()`.
-
-      This accessor is only available when the scalar type and array dimension
-      were specified as template parameters.
-
 Data types
 ^^^^^^^^^^
 

docs/ndarray.rst

@@ -33,14 +33,14 @@ Binding functions that take arrays as input
 -------------------------------------------
 
 A function that accepts a :cpp:class:`nb::ndarray\<\> <ndarray>`-typed parameter
-(i.e., *without* template parameters) can be called with *any* array
+(i.e., *without* template parameters) can be called with *any* writable array
 from any framework regardless of the device on which it is stored. The
 following example binding declaration uses this functionality to inspect the
 properties of an arbitrary input array:
 
 .. code-block:: cpp
 
-   m.def("inspect", [](nb::ndarray<> a) {
+   m.def("inspect", [](const nb::ndarray<>& a) {
        printf("Array data pointer : %p\n", a.data());
        printf("Array dimension : %zu\n", a.ndim());
        for (size_t i = 0; i < a.ndim(); ++i) {

include/nanobind/ndarray.h

@@ -110,7 +110,7 @@ template <size_t N> using ndim = typename detail::ndim_shape<std::make_index_seq
 template <typename T> constexpr dlpack::dtype dtype() {
     static_assert(
         detail::is_ndarray_scalar_v<T>,
-        "nanobind::dtype<T>: T must be a floating point or integer variable!"
+        "nanobind::dtype<T>: T must be a floating point or integer type!"
     );
 
     dlpack::dtype result;
@@ -266,46 +266,76 @@ template <typename T> struct ndarray_arg<T, enable_if_t<T::is_device>> {
 template <typename... Ts> struct ndarray_info {
     using scalar_type = void;
     using shape_type = void;
+    constexpr static bool ReadOnly = false;
     constexpr static auto name = const_name("ndarray");
     constexpr static ndarray_framework framework = ndarray_framework::none;
     constexpr static char order = '\0';
 };
 
 template <typename T, typename... Ts> struct ndarray_info<T, Ts...>  : ndarray_info<Ts...> {
     using scalar_type =
-        std::conditional_t<ndarray_traits<T>::is_float || ndarray_traits<T>::is_int ||
-                           ndarray_traits<T>::is_bool || ndarray_traits<T>::is_complex,
-                           T, typename ndarray_info<Ts...>::scalar_type>;
+        std::conditional_t<
+                detail::is_ndarray_scalar_v<T> &&
+                    std::is_void_v<typename ndarray_info<Ts...>::scalar_type>,
+                T, typename ndarray_info<Ts...>::scalar_type>;
+
+    constexpr static bool ReadOnly = ndarray_info<Ts...>::ReadOnly ||
+            (detail::is_ndarray_scalar_v<T> && std::is_const_v<T>);
+};
+
+template <typename... Ts> struct ndarray_info<ro, Ts...> : ndarray_info<Ts...> {
+    constexpr static bool ReadOnly = true;
 };
 
 template <ssize_t... Is, typename... Ts> struct ndarray_info<shape<Is...>, Ts...> : ndarray_info<Ts...> {
-    using shape_type = shape<Is...>;
+    using shape_type =
+        std::conditional_t<
+                std::is_void_v<typename ndarray_info<Ts...>::shape_type>,
+                shape<Is...>, typename ndarray_info<Ts...>::shape_type>;
 };
 
 template <typename... Ts> struct ndarray_info<c_contig, Ts...> : ndarray_info<Ts...> {
+    static_assert(ndarray_info<Ts...>::order == '\0'
+               || ndarray_info<Ts...>::order == 'C',
+                  "The order can only be set once.");
     constexpr static char order = 'C';
 };
 
 template <typename... Ts> struct ndarray_info<f_contig, Ts...> : ndarray_info<Ts...> {
+    static_assert(ndarray_info<Ts...>::order == '\0'
+               || ndarray_info<Ts...>::order == 'F',
+                  "The order can only be set once.");
     constexpr static char order = 'F';
 };
 
 template <typename... Ts> struct ndarray_info<numpy, Ts...> : ndarray_info<Ts...> {
+    static_assert(ndarray_info<Ts...>::framework == ndarray_framework::none
+               || ndarray_info<Ts...>::framework == ndarray_framework::numpy,
+                  "The framework can only be set once.");
     constexpr static auto name = const_name("numpy.ndarray");
     constexpr static ndarray_framework framework = ndarray_framework::numpy;
 };
 
 template <typename... Ts> struct ndarray_info<pytorch, Ts...> : ndarray_info<Ts...> {
+    static_assert(ndarray_info<Ts...>::framework == ndarray_framework::none
+               || ndarray_info<Ts...>::framework == ndarray_framework::pytorch,
+                  "The framework can only be set once.");
     constexpr static auto name = const_name("torch.Tensor");
     constexpr static ndarray_framework framework = ndarray_framework::pytorch;
 };
 
 template <typename... Ts> struct ndarray_info<tensorflow, Ts...> : ndarray_info<Ts...> {
+    static_assert(ndarray_info<Ts...>::framework == ndarray_framework::none
+               || ndarray_info<Ts...>::framework == ndarray_framework::tensorflow,
+                  "The framework can only be set once.");
     constexpr static auto name = const_name("tensorflow.python.framework.ops.EagerTensor");
     constexpr static ndarray_framework framework = ndarray_framework::tensorflow;
 };
 
 template <typename... Ts> struct ndarray_info<jax, Ts...> : ndarray_info<Ts...> {
+    static_assert(ndarray_info<Ts...>::framework == ndarray_framework::none
+               || ndarray_info<Ts...>::framework == ndarray_framework::jax,
+                  "The framework can only be set once.");
     constexpr static auto name = const_name("jaxlib.xla_extension.DeviceArray");
     constexpr static ndarray_framework framework = ndarray_framework::jax;
 };
@@ -314,9 +344,9 @@ template <typename... Ts> struct ndarray_info<cupy, Ts...> : ndarray_info<Ts...>
     constexpr static ndarray_framework framework = ndarray_framework::cupy;
 };
 
-
 NAMESPACE_END(detail)
 
+
 template <typename Scalar, typename Shape, char Order> struct ndarray_view {
     static constexpr size_t Dim = Shape::size;
 
@@ -380,7 +410,10 @@ template <typename... Args> class ndarray {
     template <typename...> friend class ndarray;
 
     using Info = detail::ndarray_info<Args...>;
-    using Scalar = typename Info::scalar_type;
+    static constexpr bool ReadOnly = Info::ReadOnly;
+    using Scalar = std::conditional_t<ReadOnly,
+                           std::add_const_t<typename Info::scalar_type>,
+                           typename Info::scalar_type>;
 
     ndarray() = default;
 
@@ -392,7 +425,7 @@ template <typename... Args> class ndarray {
     template <typename... Args2>
     explicit ndarray(const ndarray<Args2...> &other) : ndarray(other.m_handle) { }
 
-    ndarray(std::conditional_t<std::is_const_v<Scalar>, const void *, void *> data,
+    ndarray(std::conditional_t<ReadOnly, const void *, void *> data,
             size_t ndim,
             const size_t *shape,
             handle owner,
@@ -402,11 +435,11 @@ template <typename... Args> class ndarray {
             int32_t device_id = 0) {
         m_handle = detail::ndarray_create(
             (void *) data, ndim, shape, owner.ptr(), strides, &dtype,
-            std::is_const_v<Scalar>, device_type, device_id);
+            ReadOnly, device_type, device_id);
         m_dltensor = *detail::ndarray_inc_ref(m_handle);
     }
 
-    ndarray(std::conditional_t<std::is_const_v<Scalar>, const void *, void *> data,
+    ndarray(std::conditional_t<ReadOnly, const void *, void *> data,
             std::initializer_list<size_t> shape,
             handle owner,
             std::initializer_list<int64_t> strides = { },
@@ -420,7 +453,7 @@ template <typename... Args> class ndarray {
         m_handle = detail::ndarray_create(
             (void *) data, shape.size(), shape.begin(), owner.ptr(),
             (strides.size() == 0) ? nullptr : strides.begin(), &dtype,
-            std::is_const_v<Scalar>, device_type, device_id);
+            ReadOnly, device_type, device_id);
 
         m_dltensor = *detail::ndarray_inc_ref(m_handle);
     }
@@ -476,35 +509,26 @@ template <typename... Args> class ndarray {
     size_t itemsize() const { return ((size_t) dtype().bits + 7) / 8; }
     size_t nbytes() const { return ((size_t) dtype().bits * size() + 7) / 8; }
 
-    const Scalar *data() const {
-        return (const Scalar *)((const uint8_t *) m_dltensor.data + m_dltensor.byte_offset);
-    }
-
-    template <typename T = Scalar, std::enable_if_t<!std::is_const_v<T>, int> = 1>
-    Scalar *data() {
+    Scalar *data() const {
         return (Scalar *) ((uint8_t *) m_dltensor.data +
                            m_dltensor.byte_offset);
     }
 
-    template <typename T = Scalar,
-              std::enable_if_t<!std::is_const_v<T>, int> = 1, typename... Ts>
-    NB_INLINE auto &operator()(Ts... indices) {
+    template <typename... Ts>
+    NB_INLINE auto& operator()(Ts... indices) const {
         return *(Scalar *) ((uint8_t *) m_dltensor.data +
                             byte_offset(indices...));
     }
 
-    template <typename... Ts> NB_INLINE const auto & operator()(Ts... indices) const {
-        return *(const Scalar *) ((const uint8_t *) m_dltensor.data +
-                                  byte_offset(indices...));
-    }
-
     template <typename... Extra> NB_INLINE auto view() const {
         using Info2 = typename ndarray<Args..., Extra...>::Info;
-        using Scalar2 = typename Info2::scalar_type;
+        using Scalar2 = std::conditional_t<Info2::ReadOnly,
+                                std::add_const_t<typename Info2::scalar_type>,
+                                typename Info2::scalar_type>;
         using Shape2 = typename Info2::shape_type;
 
-        constexpr bool has_scalar = !std::is_same_v<Scalar2, void>,
-                       has_shape  = !std::is_same_v<Shape2, void>;
+        constexpr bool has_scalar = !std::is_void_v<Scalar2>,
+                       has_shape  = !std::is_void_v<Shape2>;
 
         static_assert(has_scalar,
             "To use the ndarray::view<..>() method, you must add a scalar type "
@@ -528,8 +552,8 @@ template <typename... Args> class ndarray {
 private:
     template <typename... Ts>
     NB_INLINE int64_t byte_offset(Ts... indices) const {
-        constexpr bool has_scalar = !std::is_same_v<Scalar, void>,
-                       has_shape = !std::is_same_v<typename Info::shape_type, void>;
+        constexpr bool has_scalar = !std::is_void_v<Scalar>,
+                       has_shape = !std::is_void_v<typename Info::shape_type>;
 
         static_assert(has_scalar,
             "To use ndarray::operator(), you must add a scalar type "
@@ -547,7 +571,7 @@ template <typename... Args> class ndarray {
             int64_t index = 0;
             ((index += int64_t(indices) * m_dltensor.strides[counter++]), ...);
 
-            return (int64_t) m_dltensor.byte_offset + index * sizeof(typename Info::scalar_type);
+            return (int64_t) m_dltensor.byte_offset + index * sizeof(Scalar);
         } else {
             return 0;
         }