ENH: Add itk::bit_cast<TDestination>(source) function

Added a rudimentary `bit_cast` implementation for C++14/C++17, mimicking
C++20 `std::bit_cast`.

Modules/Core/Common/include/itkBitCast.h

@@ -0,0 +1,60 @@
+/*=========================================================================
+ *
+ *  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
+ *
+ *         http://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.
+ *
+ *=========================================================================*/
+
+#ifndef itkBitCast_h
+#define itkBitCast_h
+
+#if __cplusplus < 202002L
+
+#  include <cstring>     // For memcpy.
+#  include <type_traits> // For is_trivially_copyable and remove_const_t.
+
+namespace itk
+{
+
+/** Rudimentary `bit_cast` implementation for C++14/C++17. From C++20, `std::bit_cast` is preferred.
+ */
+template <typename TDestination, class TSource>
+TDestination
+bit_cast(const TSource & source)
+{
+  static_assert(sizeof(TDestination) == sizeof(TSource),
+                "The destination type should have the same size as the source type.");
+  static_assert(std::is_trivially_copyable<TDestination>::value, "The destination type should be trivially copyable");
+  static_assert(std::is_trivially_copyable<TSource>::value, "The source type should be trivially copyable.");
+
+  std::remove_const_t<TDestination> result;
+  std::memcpy(&result, &source, sizeof(TSource));
+  return result;
+}
+
+} // namespace itk
+
+#else
+
+// From C++20, std::bit_cast is included with the C++ Standard Library.
+#  include <bit>
+
+namespace itk
+{
+using ::std::bit_cast;
+}
+
+#endif
+
+#endif // itkBitCast_h

Modules/Core/Common/test/CMakeLists.txt

@@ -626,6 +626,7 @@ endif()
 
 set(ITKCommonGTests
       itkAggregateTypesGTest.cxx
+      itkBitCastGTest.cxx
       itkBooleanStdVectorGTest.cxx
       itkBuildInformationGTest.cxx
       itkConnectedImageNeighborhoodShapeGTest.cxx

Modules/Core/Common/test/itkBitCastGTest.cxx

@@ -0,0 +1,46 @@
+/*=========================================================================
+ *
+ *  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
+ *
+ *         http://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 "itkBitCast.h"
+#include <gtest/gtest.h>
+#include <cstring> // For memcmp.
+
+namespace
+{
+template <typename TDestination, class TSource>
+void
+Expect_return_value_is_bitwise_equal_to_function_argument(const TSource & source)
+{
+  const auto result = itk::bit_cast<TDestination>(source);
+  EXPECT_EQ(std::memcmp(&result, &source, sizeof(TSource)), 0);
+}
+} // namespace
+
+
+TEST(BitCast, ResultIsBitwiseEqualToArgument)
+{
+  for (const int i : { -1, 0, 1 })
+  {
+    Expect_return_value_is_bitwise_equal_to_function_argument<unsigned int>(i);
+  }
+
+  int value;
+  (void)value;
+  Expect_return_value_is_bitwise_equal_to_function_argument<intptr_t>(&value);
+}