ENH: Add TEST(ObjectFactoryBase, CreatedInstanceHasReferenceCountOfTwo)

Tests that `ObjectFactoryBase::CreateInstance` creates an instance that has a
reference count of `2`.

Author: N-Dekker

Modules/Core/Common/test/CMakeLists.txt

@@ -672,6 +672,7 @@ set(ITKCommonGTests
       itkMersenneTwisterRandomVariateGeneratorGTest.cxx
       itkNeighborhoodAllocatorGTest.cxx
       itkNumberToStringGTest.cxx
+      itkObjectFactoryBaseGTest.cxx
       itkOffsetGTest.cxx
       itkOptimizerParametersGTest.cxx
       itkPointGTest.cxx

Modules/Core/Common/test/itkObjectFactoryBaseGTest.cxx

@@ -0,0 +1,109 @@
+/*=========================================================================
+ *
+ *  Copyright NumFOCUS
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         https://www.apache.org/licenses/LICENSE-2.0.txt
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *=========================================================================*/
+
+
+// First include the header file to be tested:
+#include "itkObjectFactoryBase.h"
+
+#include "itkLightObject.h"
+#include "itkSmartPointer.h"
+#include "itkVersion.h"
+
+#include <gtest/gtest.h>
+
+
+namespace
+{
+
+// The name of the test object type, as stored by the test object factory.
+constexpr const char * testObjectTypeName = "Test Object";
+
+// A "dummy" object type, just for unit test purposes.
+class TestObject : public itk::LightObject
+{};
+
+
+// A factory of test objects.
+class TestObjectFactory : public itk::ObjectFactoryBase
+{
+public:
+  ITK_DISALLOW_COPY_AND_MOVE(TestObjectFactory);
+
+  using Self = TestObjectFactory;
+  using Superclass = ObjectFactoryBase;
+  using Pointer = itk::SmartPointer<Self>;
+  using ConstPointer = itk::SmartPointer<const Self>;
+
+  const char *
+  GetITKSourceVersion() const override
+  {
+    return ITK_SOURCE_VERSION;
+  }
+
+  const char *
+  GetDescription() const override
+  {
+    return "Test Object Factory";
+  }
+
+  itkTypeMacro(TestObjectFactory, itk::ObjectFactoryBase);
+
+  itkFactorylessNewMacro(Self);
+
+protected:
+  TestObjectFactory()
+  {
+    this->RegisterOverride(
+      testObjectTypeName, testObjectTypeName, testObjectTypeName, true, itk::CreateObjectFunction<TestObject>::New());
+  }
+  ~TestObjectFactory() override = default;
+};
+
+
+// Ensures that a TestObjectFactory is registered and unregistered, following the C++ RAII idiom (Resource Acquisition
+// Is Initialization).
+class TestObjectFactoryRegistration
+{
+public:
+  ITK_DISALLOW_COPY_AND_MOVE(TestObjectFactoryRegistration);
+
+  TestObjectFactoryRegistration() { itk::ObjectFactoryBase::RegisterFactory(m_Factory); }
+  ~TestObjectFactoryRegistration() { itk::ObjectFactoryBase::UnRegisterFactory(m_Factory); }
+
+private:
+  TestObjectFactory::Pointer m_Factory{ TestObjectFactory::New() };
+};
+} // namespace
+
+
+// Tests that ObjectFactoryBase::CreateInstance creates an instance that has a reference count of 2.
+TEST(ObjectFactoryBase, CreatedInstanceHasReferenceCountOfTwo)
+{
+  const TestObjectFactoryRegistration registration{};
+
+  const auto instance = itk::ObjectFactoryBase::CreateInstance(testObjectTypeName);
+
+  // The type specified by testObjectTypeName is registered, so the created instance may not be null.
+  ASSERT_NE(instance, nullptr);
+
+  // The main check: expect the reference count to be 2, not 1, and certainly not zero!
+  EXPECT_EQ(instance->GetReferenceCount(), 2);
+
+  // Avoid a memory leak.
+  instance->UnRegister();
+}