ENH: Let x::New() initialize the created object by doing new x()

Replace `new x` with `new x()`, in the definitions of `itkSimpleNewMacro` and
`itkFactorylessNewMacro`. This ensures that objects created by `x::New()` are
"value-initialized". In practice, this will zero-initialize data members that
might otherwise be left uninitialized.

Added GTests to check that `x::New()` does indeed properly initialize the
created object.

Modules/Core/Common/include/itkMacro.h

@@ -312,7 +312,7 @@ namespace itk
     Pointer smartPtr = ::itk::ObjectFactory<x>::Create(); \
     if (smartPtr == nullptr)                              \
     {                                                     \
-      smartPtr = new x;                                   \
+      smartPtr = new x();                                 \
     }                                                     \
     smartPtr->UnRegister();                               \
     return smartPtr;                                      \
@@ -369,7 +369,7 @@ namespace itk
 #define itkFactorylessNewMacro(x) \
   static Pointer New()            \
   {                               \
-    x *     rawPtr = new x;       \
+    x *     rawPtr = new x();     \
     Pointer smartPtr = rawPtr;    \
     rawPtr->UnRegister();         \
     return smartPtr;              \

Modules/Core/Common/test/CMakeLists.txt

@@ -1734,6 +1734,7 @@ set(ITKCommonGTests
     itkImageRegionGTest.cxx
     itkIndexGTest.cxx
     itkIndexRangeGTest.cxx
+    itkLightObjectGTest.cxx
     itkMakeUniqueForOverwriteGTest.cxx
     itkMatrixGTest.cxx
     itkMersenneTwisterRandomVariateGeneratorGTest.cxx

Modules/Core/Common/test/itkLightObjectGTest.cxx

@@ -0,0 +1,82 @@
+/*=========================================================================
+ *
+ *  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 "itkLightObject.h"
+#include "itkObjectFactory.h"
+#include <gtest/gtest.h>
+
+
+// Tests that New() does initialize the data of a derived class, even when the derived class has a defaulted
+// default-constructor *and* its data has no {} initializer. Uses itkSimpleNewMacro to define New().
+TEST(LightObject, SimpleNewInitializesDataOfDerivedClass)
+{
+  class DerivedClass : public itk::LightObject
+  {
+  public:
+    ITK_DISALLOW_COPY_AND_MOVE(DerivedClass);
+
+    using Self = DerivedClass;
+    using Pointer = itk::SmartPointer<Self>;
+
+    itkSimpleNewMacro(DerivedClass);
+    itkGetConstMacro(Data, int);
+
+  protected:
+    // Defaulted default-constructor, essential to this test.
+    DerivedClass() = default;
+
+    // Defaulted destructor.
+    ~DerivedClass() override = default;
+
+  private:
+    int m_Data; // Without {} initializer, for the purpose of this test.
+  };
+
+  EXPECT_EQ(DerivedClass::New()->GetData(), 0);
+}
+
+
+// Tests that New() does initialize the data of a derived class, even when the derived class has a defaulted
+// default-constructor *and* its data has no {} initializer. Uses itkFactorylessNewMacro to define New().
+TEST(LightObject, FactorylessNewInitializesDataOfDerivedClass)
+{
+  class DerivedClass : public itk::LightObject
+  {
+  public:
+    ITK_DISALLOW_COPY_AND_MOVE(DerivedClass);
+
+    using Self = DerivedClass;
+    using Pointer = itk::SmartPointer<Self>;
+
+    itkFactorylessNewMacro(DerivedClass);
+    itkGetConstMacro(Data, int);
+
+  protected:
+    // Defaulted default-constructor, essential to this test.
+    DerivedClass() = default;
+
+    // Defaulted destructor.
+    ~DerivedClass() override = default;
+
+  private:
+    int m_Data; // Without {} initializer, for the purpose of this test.
+  };
+
+  EXPECT_EQ(DerivedClass::New()->GetData(), 0);
+}