BUG: Fix reference count in ConvertItkTransformBaseToSingleItkTransform

itk::ObjectFactoryBase::CreateInstance appears to return an object whose reference count is 2, while the object returned by ConvertItkTransformBaseToSingleItkTransform should have reference count 1.

Related to commit InsightSoftwareConsortium/ITK@8e3c544 "BUG: The ability to override classes with factories was not working", Bill Lorensen, November 8, 2007.

Common/Transforms/elxTransformIO.cxx

@@ -171,15 +171,20 @@ elastix::TransformIO::ConvertItkTransformBaseToSingleItkTransform(const itk::Tra
 
   const auto instanceName = className + "_double_" + std::to_string(elxTransform.GetInputSpaceDimension()) + '_' +
                             std::to_string(elxTransform.GetOutputSpaceDimension()) + optionalSplineOrderPostfix;
-  const auto instance = itk::ObjectFactoryBase::CreateInstance(instanceName.c_str());
-  const auto itkTransform = dynamic_cast<itk::TransformBase *>(instance.GetPointer());
-  if (itkTransform == nullptr)
+  if (const auto instance = itk::ObjectFactoryBase::CreateInstance(instanceName.c_str()))
   {
-    return nullptr;
+    // itk::ObjectFactoryBase::CreateInstance appears to return an object that has ReferenceCount == 2.
+    assert(instance->GetReferenceCount() == 2);
+    instance->UnRegister();
+
+    if (const auto itkTransform = dynamic_cast<itk::TransformBase *>(instance.GetPointer()))
+    {
+      itkTransform->SetFixedParameters(elxTransform.GetFixedParameters());
+      itkTransform->SetParameters(elxTransform.GetParameters());
+      return itkTransform;
+    }
   }
-  itkTransform->SetFixedParameters(elxTransform.GetFixedParameters());
-  itkTransform->SetParameters(elxTransform.GetParameters());
-  return itkTransform;
+  return nullptr;
 }
 
 

Core/Main/GTesting/itkElastixRegistrationMethodGTest.cxx

@@ -1501,7 +1501,11 @@ GTEST_TEST(itkElastixRegistrationMethod, ConvertToItkTransform)
     for (unsigned int n{ 0 }; n < numberOfTransforms; ++n)
     {
       // TODO Check result
-      ElastixRegistrationMethodType<ImageType>::ConvertToItkTransform(*registration.GetNthTransform(n));
+      const auto result =
+        ElastixRegistrationMethodType<ImageType>::ConvertToItkTransform(*registration.GetNthTransform(n));
+
+      ASSERT_NE(result, nullptr);
+      EXPECT_EQ(result->GetReferenceCount(), 1);
     }
   }
 }