BLI: improve const handling in ImplicitSharingPtr

The constness of the `ImplicitSharingPtr` does not imply the constness of the
referenced data, because that is determined by the user count. Therefore,
`ImplicitSharingPtr` should never give a non-const pointer to the underlying data.
Instead, one always has to check the user count, before one can do a `const_cast`.

Pull Request: https://projects.blender.org/blender/blender/pulls/115652

source/blender/blenkernel/intern/geometry_set.cc

@@ -120,17 +120,17 @@ GeometryComponent &GeometrySet::get_component_for_write(GeometryComponent::Type
   if (!component_ptr) {
     /* If the component did not exist before, create a new one. */
     component_ptr = GeometryComponent::create(component_type);
-    return *component_ptr;
   }
-  if (component_ptr->is_mutable()) {
+  else if (component_ptr->is_mutable()) {
     /* If the referenced component is already mutable, return it directly. */
     component_ptr->tag_ensured_mutable();
-    return *component_ptr;
   }
-  /* If the referenced component is shared, make a copy. The copy is not shared and is
-   * therefore mutable. */
-  component_ptr = GeometryComponentPtr(component_ptr->copy());
-  return *component_ptr;
+  else {
+    /* If the referenced component is shared, make a copy. The copy is not shared and is
+     * therefore mutable. */
+    component_ptr = GeometryComponentPtr(component_ptr->copy());
+  }
+  return const_cast<GeometryComponent &>(*component_ptr);
 }
 
 GeometryComponent *GeometrySet::get_component_ptr(GeometryComponent::Type type)

source/blender/blenlib/BLI_implicit_sharing_ptr.hh

@@ -20,12 +20,12 @@ namespace blender {
  */
 template<typename T> class ImplicitSharingPtr {
  private:
-  T *data_ = nullptr;
+  const T *data_ = nullptr;
 
  public:
   ImplicitSharingPtr() = default;
 
-  explicit ImplicitSharingPtr(T *data) : data_(data) {}
+  explicit ImplicitSharingPtr(const T *data) : data_(data) {}
 
   /* Implicit conversion from nullptr. */
   ImplicitSharingPtr(std::nullptr_t) : data_(nullptr) {}
@@ -69,24 +69,12 @@ template<typename T> class ImplicitSharingPtr {
     return *this;
   }
 
-  T *operator->()
-  {
-    BLI_assert(data_ != nullptr);
-    return data_;
-  }
-
   const T *operator->() const
   {
     BLI_assert(data_ != nullptr);
     return data_;
   }
 
-  T &operator*()
-  {
-    BLI_assert(data_ != nullptr);
-    return *data_;
-  }
-
   const T &operator*() const
   {
     BLI_assert(data_ != nullptr);
@@ -98,17 +86,12 @@ template<typename T> class ImplicitSharingPtr {
     return data_ != nullptr;
   }
 
-  T *get()
-  {
-    return data_;
-  }
-
   const T *get() const
   {
     return data_;
   }
 
-  T *release()
+  const T *release()
   {
     T *data = data_;
     data_ = nullptr;
@@ -134,14 +117,14 @@ template<typename T> class ImplicitSharingPtr {
   BLI_STRUCT_EQUALITY_OPERATORS_1(ImplicitSharingPtr, data_)
 
  private:
-  static void add_user(T *data)
+  static void add_user(const T *data)
   {
     if (data != nullptr) {
       data->add_user();
     }
   }
 
-  static void remove_user_and_delete_if_last(T *data)
+  static void remove_user_and_delete_if_last(const T *data)
   {
     if (data != nullptr) {
       data->remove_user_and_delete_if_last();

source/blender/blenlib/tests/BLI_implicit_sharing_test.cc

@@ -47,10 +47,11 @@ class SharedDataContainer {
     }
     if (data_->is_mutable()) {
       data_->tag_ensured_mutable();
-      return data_.get();
     }
-    data_ = data_->copy();
-    return data_.get();
+    else {
+      data_ = data_->copy();
+    }
+    return const_cast<ImplicitlySharedData *>(data_.get());
   }
 };