Use std::optional<void*> for custom_data instead of raw pointer

Replace void* with nullptr pattern with std::optional<void*> for
custom_data in the Form class and assembly functions. This is the
canonical modern C++ approach for representing optional values.

Changes:
- Form.h: integral_data::custom_data now uses std::optional<void*>
- Form.h: custom_data() returns std::optional<void*>
- Form.h: set_custom_data() accepts std::optional<void*>
- assemble_*_impl.h: Function parameters use std::optional<void*>
- assemble_*_impl.h: Kernel calls use .value_or(nullptr)
- Python bindings: Handle std::optional with None support
- Test: Update to expect None instead of 0 for unset custom_data

Benefits:
- Clearer intent: "optional value" vs "magic nullptr"
- Type safety: Distinguishes "no value" from "null pointer value"
- Pythonic: Returns None instead of 0 when not set
- Zero-cost: .value_or(nullptr) compiles to same code

Author: sclaus2

cpp/dolfinx/fem/Form.h

@@ -67,7 +67,7 @@ struct integral_data
                  and std::is_convertible_v<std::remove_cvref_t<W>,
                                            std::vector<int>>
   integral_data(K&& kernel, V&& entities, W&& coeffs,
-                void* custom_data = nullptr)
+                std::optional<void*> custom_data = std::nullopt)
       : kernel(std::forward<K>(kernel)), entities(std::forward<V>(entities)),
         coeffs(std::forward<W>(coeffs)), custom_data(custom_data)
   {
@@ -89,7 +89,7 @@ struct integral_data
   /// @brief Custom user data pointer passed to the kernel function.
   /// This can be used to pass runtime-computed data (e.g., per-cell
   /// quadrature rules, material properties) to the kernel.
-  void* custom_data = nullptr;
+  std::optional<void*> custom_data = std::nullopt;
 };
 
 /// @brief A representation of finite element variational forms.
@@ -409,8 +409,8 @@ class Form
   /// @param[in] id Integral subdomain ID.
   /// @param[in] kernel_idx Index of the kernel (we may have multiple
   /// kernels for a given ID in mixed-topology meshes).
-  /// @return Custom data pointer for the integral, or nullptr if not set.
-  void* custom_data(IntegralType type, int id, int kernel_idx) const
+  /// @return Custom data pointer for the integral, or std::nullopt if not set.
+  std::optional<void*> custom_data(IntegralType type, int id, int kernel_idx) const
   {
     auto it = _integrals.find({type, id, kernel_idx});
     if (it == _integrals.end())
@@ -423,8 +423,9 @@ class Form
   /// @param[in] type Integral type.
   /// @param[in] id Integral subdomain ID.
   /// @param[in] kernel_idx Index of the kernel.
-  /// @param[in] data Custom data pointer to set.
-  void set_custom_data(IntegralType type, int id, int kernel_idx, void* data)
+  /// @param[in] data Custom data pointer to set, or std::nullopt to clear.
+  void set_custom_data(IntegralType type, int id, int kernel_idx,
+                       std::optional<void*> data)
   {
     auto it = _integrals.find({type, id, kernel_idx});
     if (it == _integrals.end())

cpp/dolfinx/fem/assemble_matrix_impl.h

@@ -18,6 +18,7 @@
 #include <dolfinx/mesh/Topology.h>
 #include <functional>
 #include <iterator>
+#include <optional>
 #include <span>
 #include <tuple>
 #include <vector>
@@ -75,7 +76,8 @@ void assemble_cells_matrix(
     std::span<const std::int8_t> bc1, FEkernel<T> auto kernel,
     md::mdspan<const T, md::dextents<std::size_t, 2>> coeffs,
     std::span<const T> constants, std::span<const std::uint32_t> cell_info0,
-    std::span<const std::uint32_t> cell_info1, void* custom_data = nullptr)
+    std::span<const std::uint32_t> cell_info1,
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (cells.empty())
     return;
@@ -110,7 +112,7 @@ void assemble_cells_matrix(
     // Tabulate tensor
     std::ranges::fill(Ae, 0);
     kernel(Ae.data(), &coeffs(c, 0), constants.data(), cdofs.data(), nullptr,
-           nullptr, custom_data);
+           nullptr, custom_data.value_or(nullptr));
 
     // Compute A = P_0 \tilde{A} P_1^T (dof transformation)
     P0(Ae, cell_info0, cell0, ndim1);  // B = P0 \tilde{A}
@@ -224,7 +226,7 @@ void assemble_entities(
     std::span<const T> constants, std::span<const std::uint32_t> cell_info0,
     std::span<const std::uint32_t> cell_info1,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (entities.empty())
     return;
@@ -262,7 +264,7 @@ void assemble_entities(
     // Tabulate tensor
     std::ranges::fill(Ae, 0);
     kernel(Ae.data(), &coeffs(f, 0), constants.data(), cdofs.data(),
-           &local_entity, &perm, custom_data);
+           &local_entity, &perm, custom_data.value_or(nullptr));
     P0(Ae, cell_info0, cell0, ndim1);
     P1T(Ae, cell_info1, cell1, ndim0);
 
@@ -367,7 +369,7 @@ void assemble_interior_facets(
     std::span<const T> constants, std::span<const std::uint32_t> cell_info0,
     std::span<const std::uint32_t> cell_info1,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (facets.empty())
     return;
@@ -444,7 +446,7 @@ void assemble_interior_facets(
                           : std::array{perms(cells[0], local_facet[0]),
                                        perms(cells[1], local_facet[1])};
     kernel(Ae.data(), &coeffs(f, 0, 0), constants.data(), cdofs.data(),
-           local_facet.data(), perm.data(), custom_data);
+           local_facet.data(), perm.data(), custom_data.value_or(nullptr));
 
     // Local element layout is a 2x2 block matrix with structure
     //
@@ -609,7 +611,8 @@ void assemble_matrix(
       std::span cells0 = a.domain_arg(IntegralType::cell, 0, i, cell_type_idx);
       std::span cells1 = a.domain_arg(IntegralType::cell, 1, i, cell_type_idx);
       auto& [coeffs, cstride] = coefficients.at({IntegralType::cell, i});
-      void* custom_data = a.custom_data(IntegralType::cell, i, cell_type_idx);
+      std::optional<void*> custom_data
+          = a.custom_data(IntegralType::cell, i, cell_type_idx);
       assert(cells.size() * cstride == coeffs.size());
       impl::assemble_cells_matrix(
           mat_set, x_dofmap, x, cells, {dofs0, bs0, cells0}, P0,
@@ -651,7 +654,8 @@ void assemble_matrix(
       assert(fn);
       auto& [coeffs, cstride]
           = coefficients.at({IntegralType::interior_facet, i});
-      void* custom_data = a.custom_data(IntegralType::interior_facet, i, 0);
+      std::optional<void*> custom_data
+          = a.custom_data(IntegralType::interior_facet, i, 0);
 
       std::span facets = a.domain(IntegralType::interior_facet, i, 0);
       std::span facets0 = a.domain_arg(IntegralType::interior_facet, 0, i, 0);
@@ -694,7 +698,7 @@ void assemble_matrix(
         auto fn = a.kernel(itg_type, i, 0);
         assert(fn);
         auto& [coeffs, cstride] = coefficients.at({itg_type, i});
-        void* custom_data = a.custom_data(itg_type, i, 0);
+        std::optional<void*> custom_data = a.custom_data(itg_type, i, 0);
 
         std::span e = a.domain(itg_type, i, 0);
         mdspanx2_t entities(e.data(), e.size() / 2, 2);

cpp/dolfinx/fem/assemble_scalar_impl.h

@@ -17,6 +17,7 @@
 #include <dolfinx/mesh/Mesh.h>
 #include <dolfinx/mesh/Topology.h>
 #include <memory>
+#include <optional>
 #include <vector>
 
 namespace dolfinx::fem::impl
@@ -31,7 +32,7 @@ T assemble_cells(mdspan2_t x_dofmap,
                  std::span<const T> constants,
                  md::mdspan<const T, md::dextents<std::size_t, 2>> coeffs,
                  std::span<scalar_value_t<T>> cdofs_b,
-                 void* custom_data = nullptr)
+                 std::optional<void*> custom_data = std::nullopt)
 {
   T value(0);
   if (cells.empty())
@@ -50,7 +51,7 @@ T assemble_cells(mdspan2_t x_dofmap,
       std::copy_n(&x(x_dofs[i], 0), 3, std::next(cdofs_b.begin(), 3 * i));
 
     fn(&value, &coeffs(index, 0), constants.data(), cdofs_b.data(), nullptr,
-       nullptr, custom_data);
+       nullptr, custom_data.value_or(nullptr));
   }
 
   return value;
@@ -78,7 +79,7 @@ T assemble_entities(
     FEkernel<T> auto fn, std::span<const T> constants,
     md::mdspan<const T, md::dextents<std::size_t, 2>> coeffs,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    std::span<scalar_value_t<T>> cdofs_b, void* custom_data = nullptr)
+    std::span<scalar_value_t<T>> cdofs_b, std::optional<void*> custom_data = std::nullopt)
 {
   T value(0);
   if (entities.empty())
@@ -100,7 +101,7 @@ T assemble_entities(
     // Permutations
     std::uint8_t perm = perms.empty() ? 0 : perms(cell, local_entity);
     fn(&value, &coeffs(f, 0), constants.data(), cdofs_b.data(), &local_entity,
-       &perm, custom_data);
+       &perm, custom_data.value_or(nullptr));
   }
 
   return value;
@@ -121,7 +122,7 @@ T assemble_interior_facets(
                                     md::dynamic_extent>>
         coeffs,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    std::span<scalar_value_t<T>> cdofs_b, void* custom_data = nullptr)
+    std::span<scalar_value_t<T>> cdofs_b, std::optional<void*> custom_data = std::nullopt)
 {
   T value(0);
   if (facets.empty())
@@ -151,7 +152,7 @@ T assemble_interior_facets(
                           : std::array{perms(cells[0], local_facet[0]),
                                        perms(cells[1], local_facet[1])};
     fn(&value, &coeffs(f, 0, 0), constants.data(), cdofs_b.data(),
-       local_facet.data(), perm.data(), custom_data);
+       local_facet.data(), perm.data(), custom_data.value_or(nullptr));
   }
 
   return value;
@@ -179,7 +180,7 @@ T assemble_scalar(
     auto fn = M.kernel(IntegralType::cell, i, 0);
     assert(fn);
     auto& [coeffs, cstride] = coefficients.at({IntegralType::cell, i});
-    void* custom_data = M.custom_data(IntegralType::cell, i, 0);
+    std::optional<void*> custom_data = M.custom_data(IntegralType::cell, i, 0);
     std::span<const std::int32_t> cells = M.domain(IntegralType::cell, i, 0);
     assert(cells.size() * cstride == coeffs.size());
     value += impl::assemble_cells(
@@ -206,7 +207,7 @@ T assemble_scalar(
     assert(fn);
     auto& [coeffs, cstride]
         = coefficients.at({IntegralType::interior_facet, i});
-    void* custom_data = M.custom_data(IntegralType::interior_facet, i, 0);
+    std::optional<void*> custom_data = M.custom_data(IntegralType::interior_facet, i, 0);
     std::span facets = M.domain(IntegralType::interior_facet, i, 0);
 
     constexpr std::size_t num_adjacent_cells = 2;
@@ -239,7 +240,7 @@ T assemble_scalar(
       auto fn = M.kernel(itg_type, i, 0);
       assert(fn);
       auto& [coeffs, cstride] = coefficients.at({itg_type, i});
-      void* custom_data = M.custom_data(itg_type, i, 0);
+      std::optional<void*> custom_data = M.custom_data(itg_type, i, 0);
 
       std::span entities = M.domain(itg_type, i, 0);
 

cpp/dolfinx/fem/assemble_vector_impl.h

@@ -93,7 +93,7 @@ void _lift_bc_cells(
     std::span<const std::uint32_t> cell_info0,
     std::span<const std::uint32_t> cell_info1, std::span<const T> bc_values1,
     std::span<const std::int8_t> bc_markers1, std::span<const T> x0, T alpha,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (cells.empty())
     return;
@@ -165,7 +165,7 @@ void _lift_bc_cells(
 
     std::ranges::fill(Ae, 0);
     kernel(Ae.data(), &coeffs(index, 0), constants.data(), cdofs.data(),
-           nullptr, nullptr, custom_data);
+           nullptr, nullptr, custom_data.value_or(nullptr));
     P0(Ae, cell_info0, c0, num_cols);
     P1T(Ae, cell_info1, c1, num_rows);
 
@@ -288,7 +288,7 @@ void _lift_bc_entities(
     std::span<const std::uint32_t> cell_info1, std::span<const T> bc_values1,
     std::span<const std::int8_t> bc_markers1, std::span<const T> x0, T alpha,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (entities.empty())
     return;
@@ -347,7 +347,7 @@ void _lift_bc_entities(
     std::uint8_t perm = perms.empty() ? 0 : perms(cell, local_entity);
     std::ranges::fill(Ae, 0);
     kernel(Ae.data(), &coeffs(index, 0), constants.data(), cdofs.data(),
-           &local_entity, &perm, custom_data);
+           &local_entity, &perm, custom_data.value_or(nullptr));
     P0(Ae, cell_info0, cell0, num_cols);
     P1T(Ae, cell_info1, cell1, num_rows);
 
@@ -446,7 +446,7 @@ void _lift_bc_interior_facets(
     std::span<const std::uint32_t> cell_info1, std::span<const T> bc_values1,
     std::span<const std::int8_t> bc_markers1, std::span<const T> x0, T alpha,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (facets.empty())
     return;
@@ -562,7 +562,7 @@ void _lift_bc_interior_facets(
                           : std::array{perms(cells[0], local_facet[0]),
                                        perms(cells[1], local_facet[1])};
     kernel(Ae.data(), &coeffs(f, 0, 0), constants.data(), cdofs.data(),
-           local_facet.data(), perm.data(), custom_data);
+           local_facet.data(), perm.data(), custom_data.value_or(nullptr));
 
     if (cells0[0] >= 0)
       P0(Ae, cell_info0, cells0[0], num_cols);
@@ -674,7 +674,7 @@ void assemble_cells(
     std::tuple<mdspan2_t, int, std::span<const std::int32_t>> dofmap,
     FEkernel<T> auto kernel, std::span<const T> constants,
     md::mdspan<const T, md::dextents<std::size_t, 2>> coeffs,
-    std::span<const std::uint32_t> cell_info0, void* custom_data = nullptr)
+    std::span<const std::uint32_t> cell_info0, std::optional<void*> custom_data = std::nullopt)
 {
   if (cells.empty())
     return;
@@ -701,7 +701,7 @@ void assemble_cells(
     // Tabulate vector for cell
     std::ranges::fill(be, 0);
     kernel(be.data(), &coeffs(index, 0), constants.data(), cdofs.data(),
-           nullptr, nullptr, custom_data);
+           nullptr, nullptr, custom_data.value_or(nullptr));
     P0(be, cell_info0, c0, 1);
 
     // Scatter cell vector to 'global' vector array
@@ -773,7 +773,7 @@ void assemble_entities(
     md::mdspan<const T, md::dextents<std::size_t, 2>> coeffs,
     std::span<const std::uint32_t> cell_info0,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   if (entities.empty())
     return;
@@ -805,7 +805,7 @@ void assemble_entities(
     // Tabulate element vector
     std::ranges::fill(be, 0);
     kernel(be.data(), &coeffs(f, 0), constants.data(), cdofs.data(),
-           &local_entity, &perm, custom_data);
+           &local_entity, &perm, custom_data.value_or(nullptr));
     P0(be, cell_info0, cell0, 1);
 
     // Add element vector to global vector
@@ -871,7 +871,7 @@ void assemble_interior_facets(
         coeffs,
     std::span<const std::uint32_t> cell_info0,
     md::mdspan<const std::uint8_t, md::dextents<std::size_t, 2>> perms,
-    void* custom_data = nullptr)
+    std::optional<void*> custom_data = std::nullopt)
 {
   using X = scalar_value_t<T>;
 
@@ -923,7 +923,7 @@ void assemble_interior_facets(
                           : std::array{perms(cells[0], local_facet[0]),
                                        perms(cells[1], local_facet[1])};
     kernel(be.data(), &coeffs(f, 0, 0), constants.data(), cdofs.data(),
-           local_facet.data(), perm.data(), custom_data);
+           local_facet.data(), perm.data(), custom_data.value_or(nullptr));
 
     if (cells0[0] >= 0)
       P0(be, cell_info0, cells0[0], 1);
@@ -1031,7 +1031,7 @@ void lift_bc(V&& b, const Form<T, U>& a, mdspan2_t x_dofmap,
     auto kernel = a.kernel(IntegralType::cell, i, 0);
     assert(kernel);
     auto& [_coeffs, cstride] = coefficients.at({IntegralType::cell, i});
-    void* custom_data = a.custom_data(IntegralType::cell, i, 0);
+    std::optional<void*> custom_data = a.custom_data(IntegralType::cell, i, 0);
     std::span cells = a.domain(IntegralType::cell, i, 0);
     std::span cells0 = a.domain_arg(IntegralType::cell, 0, i, 0);
     std::span cells1 = a.domain_arg(IntegralType::cell, 1, i, 0);
@@ -1079,7 +1079,7 @@ void lift_bc(V&& b, const Form<T, U>& a, mdspan2_t x_dofmap,
     assert(kernel);
     auto& [coeffs, cstride]
         = coefficients.at({IntegralType::interior_facet, i});
-    void* custom_data = a.custom_data(IntegralType::interior_facet, i, 0);
+    std::optional<void*> custom_data = a.custom_data(IntegralType::interior_facet, i, 0);
 
     using mdspanx22_t
         = md::mdspan<const std::int32_t,
@@ -1114,7 +1114,7 @@ void lift_bc(V&& b, const Form<T, U>& a, mdspan2_t x_dofmap,
       auto kernel = a.kernel(itg_type, i, 0);
       assert(kernel);
       auto& [coeffs, cstride] = coefficients.at({itg_type, i});
-      void* custom_data = a.custom_data(itg_type, i, 0);
+      std::optional<void*> custom_data = a.custom_data(itg_type, i, 0);
 
       using mdspanx2_t
           = md::mdspan<const std::int32_t,
@@ -1286,7 +1286,7 @@ void assemble_vector(
       std::span cells = L.domain(IntegralType::cell, i, cell_type_idx);
       std::span cells0 = L.domain_arg(IntegralType::cell, 0, i, cell_type_idx);
       auto& [coeffs, cstride] = coefficients.at({IntegralType::cell, i});
-      void* custom_data = L.custom_data(IntegralType::cell, i, cell_type_idx);
+      void* custom_data = L.custom_data(IntegralType::cell, i, cell_type_idx).value_or(nullptr);
       assert(cells.size() * cstride == coeffs.size());
       if (bs == 1)
       {
@@ -1341,7 +1341,7 @@ void assemble_vector(
       assert(fn);
       auto& [coeffs, cstride]
           = coefficients.at({IntegralType::interior_facet, i});
-      void* custom_data = L.custom_data(IntegralType::interior_facet, i, 0);
+      void* custom_data = L.custom_data(IntegralType::interior_facet, i, 0).value_or(nullptr);
       std::span facets = L.domain(IntegralType::interior_facet, i, 0);
       std::span facets1 = L.domain_arg(IntegralType::interior_facet, 0, i, 0);
       assert((facets.size() / 4) * 2 * cstride == coeffs.size());
@@ -1393,7 +1393,7 @@ void assemble_vector(
         auto fn = L.kernel(itg_type, i, 0);
         assert(fn);
         auto& [coeffs, cstride] = coefficients.at({itg_type, i});
-        void* custom_data = L.custom_data(itg_type, i, 0);
+        void* custom_data = L.custom_data(itg_type, i, 0).value_or(nullptr);
         std::span e = L.domain(itg_type, i, 0);
         mdspanx2_t entities(e.data(), e.size() / 2, 2);
         std::span e1 = L.domain_arg(itg_type, 0, i, 0);