memory: Add constructor for empty device_unique_object

src/stdgpu/impl/memory_detail.h

@@ -433,6 +433,12 @@ copyDevice2DeviceArray(const T* source_device_array,
 namespace stdgpu
 {
 
+template <typename T>
+device_unique_object<T>::device_unique_object(null_object_t /*null_object*/)
+  : _object(nullptr)
+{
+}
+
 template <typename T>
 template <typename... Args>
 device_unique_object<T>::device_unique_object(Args&&... args)
@@ -484,6 +490,12 @@ device_unique_object<T>::operator*()
     return *_object;
 }
 
+template <typename T>
+device_unique_object<T>::operator bool() const
+{
+    return _object.operator bool();
+}
+
 template <typename T>
 template <typename U>
 safe_device_allocator<T>::safe_device_allocator([[maybe_unused]] const safe_device_allocator<U>& other) noexcept

src/stdgpu/memory.h

@@ -263,6 +263,23 @@ copyDevice2DeviceArray(const T* source_device_array,
 namespace stdgpu
 {
 
+/**
+ * \ingroup memory
+ * \brief A type to indicate an uninitialized unique object
+ */
+struct null_object_t
+{
+    //! @cond Doxygen_Suppress
+    constexpr explicit null_object_t(int /* unspecified */) { }
+    //! @endcond
+};
+
+/**
+ * \ingroup memory
+ * \brief A constant to indicate an uninitialized unique object
+ */
+inline constexpr null_object_t null_object{ 0 };
+
 /**
  * \ingroup memory
  * \brief A resource wrapper for managing device objects with automatic scope-based object destruction
@@ -272,6 +289,11 @@ template <typename T>
 class device_unique_object
 {
 public:
+    /**
+     * \brief Creates an empty unique object
+     */
+    explicit device_unique_object(null_object_t /*null_object*/);
+
     /**
      * \brief Creates an object on the GPU (device)
      * \tparam Args The argument types
@@ -320,6 +342,13 @@ class device_unique_object
     T&
     operator*();
 
+    /**
+     * \brief Checks whether the unique object is not empty
+     * \return True if the unique object is not empty, false otherwise
+     */
+    explicit
+    operator bool() const;
+
 private:
     std::unique_ptr<T, std::function<void(T*)>> _object;
 };

tests/stdgpu/memory.inc

@@ -1296,6 +1296,32 @@ device_allocation_balance()
 
 } // namespace
 
+TEST_F(STDGPU_MEMORY_TEST_CLASS, device_unique_object_empty)
+{
+    using T = TestContainer<float>;
+
+    const stdgpu::index64_t old_balance = device_allocation_balance();
+
+    const stdgpu::index_t old_standard_signature_used = T::standard_signature_used;
+    const stdgpu::index_t old_policy_signature_used = T::policy_signature_used;
+    EXPECT_EQ(T::standard_signature_used % 2, 0);
+    EXPECT_EQ(T::policy_signature_used % 2, 0);
+
+    {
+        [[maybe_unused]] auto object = stdgpu::device_unique_object<T>(stdgpu::null_object);
+
+        EXPECT_FALSE(object);
+        EXPECT_EQ(T::standard_signature_used, old_standard_signature_used);
+        EXPECT_EQ(T::policy_signature_used, old_policy_signature_used);
+    }
+
+    const stdgpu::index64_t new_balance = device_allocation_balance();
+    EXPECT_EQ(old_balance, new_balance);
+
+    EXPECT_EQ(T::standard_signature_used, old_standard_signature_used);
+    EXPECT_EQ(T::policy_signature_used, old_policy_signature_used);
+}
+
 TEST_F(STDGPU_MEMORY_TEST_CLASS, device_unique_object)
 {
     using T = TestContainer<float>;
@@ -1310,6 +1336,7 @@ TEST_F(STDGPU_MEMORY_TEST_CLASS, device_unique_object)
 
         [[maybe_unused]] auto object = stdgpu::device_unique_object<T>(N);
 
+        EXPECT_TRUE(object);
         EXPECT_EQ(T::standard_signature_used % 2, 1);
         EXPECT_EQ(T::policy_signature_used, old_policy_signature_used);
     }
@@ -1336,6 +1363,7 @@ TEST_F(STDGPU_MEMORY_TEST_CLASS, device_unique_object_custom_execution_policy)
 
         [[maybe_unused]] auto object = stdgpu::device_unique_object<T>(policy, N);
 
+        EXPECT_TRUE(object);
         EXPECT_EQ(T::policy_signature_used % 2, 1);
         EXPECT_EQ(T::standard_signature_used, old_standard_signature_used);
     }