Add porting layer for future std features

Currently, VTK-m is using C++11. However, it is often useful to use
features in the `std` namespace that are defined for C++14 or later. We
can provide our own versions (sometimes), but it is preferable to use
the version provided by the compiler if available.

There were already some examples of defining portable versions of C++14
and C++17 classes in a `vtkmstd` namespace, but these were sprinkled
around the source code.

There is now a top level `vtkmstd` directory and in it are header files
that provide portable versions of these future C++ classes. In each
case, preprocessor macros are used to select which version of the class
to use.

CMakeLists.txt

@@ -202,6 +202,7 @@ check_type_size("long long" VTKm_SIZE_LONG_LONG BUILTIN_TYPES_ONLY)
 
 #-----------------------------------------------------------------------------
 # Add subdirectories
+add_subdirectory(vtkmstd)
 add_subdirectory(vtkm)
 
 #-----------------------------------------------------------------------------

docs/changelog/std_porting_headers.md

@@ -0,0 +1,14 @@
+# Porting layer for future std features
+
+Currently, VTK-m is using C++11. However, it is often useful to use
+features in the `std` namespace that are defined for C++14 or later. We can
+provide our own versions (sometimes), but it is preferable to use the
+version provided by the compiler if available.
+
+There were already some examples of defining portable versions of C++14 and
+C++17 classes in a `vtkmstd` namespace, but these were sprinkled around the
+source code.
+
+There is now a top level `vtkmstd` directory and in it are header files
+that provide portable versions of these future C++ classes. In each case,
+preprocessor macros are used to select which version of the class to use.

vtkm/cont/ArrayPortalToIterators.h

@@ -15,30 +15,7 @@
 
 #include <vtkm/internal/ArrayPortalHelpers.h>
 
-namespace vtkmstd
-{
-/// Implementation of std::void_t (C++17):
-/// Allows for specialization of class templates based on members of template
-/// parameters.
-#if defined(VTKM_GCC) && (__GNUC__ < 5)
-// Due to a defect in the wording (CWG 1558) unused parameters in alias templates
-// were not guaranteed to ensure SFINAE, and therefore would consider everything
-// to match the 'true' side. For VTK-m the only known compiler that implemented
-// this defect is GCC < 5.
-template <class... T>
-struct void_pack
-{
-  using type = void;
-};
-template <class... T>
-using void_t = typename void_pack<T...>::type;
-#else
-template <typename...>
-using void_t = void;
-#endif
-
-
-} // end namespace vtkmstd
+#include <vtkmstd/void_t.h>
 
 namespace vtkm
 {

vtkm/internal/Variant.h

@@ -15,67 +15,8 @@
 #include <vtkm/Deprecated.h>
 #include <vtkm/List.h>
 
-#if defined(VTKM_USING_GLIBCXX_4)
-// It would make sense to put this in its own header file, but it is hard to imagine needing
-// aligned_union anywhere else.
-#include <algorithm>
-namespace vtkmstd
-{
-
-template <std::size_t... Xs>
-struct max_size;
-template <std::size_t X>
-struct max_size<X>
-{
-  static constexpr std::size_t value = X;
-};
-template <std::size_t X0, std::size_t... Xs>
-struct max_size<X0, Xs...>
-{
-  static constexpr std::size_t other_value = max_size<Xs...>::value;
-  static constexpr std::size_t value = (other_value > X0) ? other_value : X0;
-};
-
-// This is to get around an apparent bug in GCC 4.8 where alianas(x) does not
-// seem to work when x is a constexpr. See
-// https://stackoverflow.com/questions/29879609/g-complains-constexpr-function-is-not-a-constant-expression
-template <std::size_t Alignment, std::size_t Size>
-struct aligned_data_block
-{
-  alignas(Alignment) char _s[Size];
-};
-
-template <std::size_t Len, class... Types>
-struct aligned_union
-{
-  static constexpr std::size_t alignment_value = vtkmstd::max_size<alignof(Types)...>::value;
-
-  using type =
-    vtkmstd::aligned_data_block<alignment_value, vtkmstd::max_size<Len, sizeof(Types)...>::value>;
-};
-
-// GCC 4.8 and 4.9 standard library does not support std::is_trivially_copyable.
-// There is no relyable way to get this information (since it has to come special from
-// the compiler). For our purposes, we will report as nothing being trivially copyable,
-// which causes us to call the constructors with everything. This should be fine unless
-// some other part of the compiler is trying to check for trivial copies (perhaps nvcc
-// on top of GCC 4.8).
-template <typename>
-struct is_trivially_copyable : std::false_type
-{
-};
-
-} // namespace vtkmstd
-
-#else // NOT VTKM_USING_GLIBCXX_4
-namespace vtkmstd
-{
-
-using std::aligned_union;
-using std::is_trivially_copyable;
-
-} // namespace vtkmstd
-#endif
+#include <vtkmstd/aligned_union.h>
+#include <vtkmstd/is_trivially_copyable.h>
 
 namespace vtkm
 {

vtkmstd/CMakeLists.txt

@@ -0,0 +1,17 @@
+##============================================================================
+##  Copyright (c) Kitware, Inc.
+##  All rights reserved.
+##  See LICENSE.txt for details.
+##
+##  This software is distributed WITHOUT ANY WARRANTY; without even
+##  the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+##  PURPOSE.  See the above copyright notice for more information.
+##============================================================================
+
+set(headers
+  aligned_union.h
+  is_trivially_copyable.h
+  void_t.h
+  )
+
+vtkm_declare_headers(${headers})

vtkmstd/aligned_union.h

@@ -0,0 +1,69 @@
+//============================================================================
+//  Copyright (c) Kitware, Inc.
+//  All rights reserved.
+//  See LICENSE.txt for details.
+//
+//  This software is distributed WITHOUT ANY WARRANTY; without even
+//  the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+//  PURPOSE.  See the above copyright notice for more information.
+//============================================================================
+#ifndef vtk_m_std_aligned_union_h
+#define vtk_m_std_aligned_union_h
+
+#include <vtkm/internal/Configure.h>
+
+#include <type_traits>
+
+#if defined(VTKM_USING_GLIBCXX_4)
+
+#include <algorithm>
+
+namespace vtkmstd
+{
+
+template <std::size_t... Xs>
+struct max_size;
+template <std::size_t X>
+struct max_size<X>
+{
+  static constexpr std::size_t value = X;
+};
+template <std::size_t X0, std::size_t... Xs>
+struct max_size<X0, Xs...>
+{
+  static constexpr std::size_t other_value = max_size<Xs...>::value;
+  static constexpr std::size_t value = (other_value > X0) ? other_value : X0;
+};
+
+// This is to get around an apparent bug in GCC 4.8 where alianas(x) does not
+// seem to work when x is a constexpr. See
+// https://stackoverflow.com/questions/29879609/g-complains-constexpr-function-is-not-a-constant-expression
+template <std::size_t Alignment, std::size_t Size>
+struct aligned_data_block
+{
+  alignas(Alignment) char _s[Size];
+};
+
+template <std::size_t Len, class... Types>
+struct aligned_union
+{
+  static constexpr std::size_t alignment_value = vtkmstd::max_size<alignof(Types)...>::value;
+
+  using type =
+    vtkmstd::aligned_data_block<alignment_value, vtkmstd::max_size<Len, sizeof(Types)...>::value>;
+};
+
+} // namespace vtkmstd
+
+#else // NOT VTKM_USING_GLIBCXX_4
+
+namespace vtkmstd
+{
+
+using std::aligned_union;
+
+} // namespace vtkmstd
+
+#endif // NOT VTKM_USING_GLIBCXX_4
+
+#endif //vtk_m_std_aligned_union_h

vtkmstd/is_trivially_copyable.h

@@ -0,0 +1,41 @@
+//============================================================================
+//  Copyright (c) Kitware, Inc.
+//  All rights reserved.
+//  See LICENSE.txt for details.
+//
+//  This software is distributed WITHOUT ANY WARRANTY; without even
+//  the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+//  PURPOSE.  See the above copyright notice for more information.
+//============================================================================
+#ifndef vtk_m_std_is_trivially_copyable_h
+#define vtk_m_std_is_trivially_copyable_h
+
+#include <type_traits>
+
+#if defined(VTKM_USING_GLIBCXX_4)
+namespace vtkmstd
+{
+
+// GCC 4.8 and 4.9 standard library does not support std::is_trivially_copyable.
+// There is no relyable way to get this information (since it has to come special from
+// the compiler). For our purposes, we will report as nothing being trivially copyable,
+// which causes us to call the constructors with everything. This should be fine unless
+// some other part of the compiler is trying to check for trivial copies (perhaps nvcc
+// on top of GCC 4.8).
+template <typename>
+struct is_trivially_copyable : std::false_type
+{
+};
+
+} // namespace vtkmstd
+
+#else // NOT VTKM_USING_GLIBCXX_4
+namespace vtkmstd
+{
+
+using std::is_trivially_copyable;
+
+} // namespace vtkmstd
+#endif
+
+#endif //vtk_m_std_is_trivially_copyable_h

vtkmstd/void_t.h

@@ -0,0 +1,40 @@
+//============================================================================
+//  Copyright (c) Kitware, Inc.
+//  All rights reserved.
+//  See LICENSE.txt for details.
+//
+//  This software is distributed WITHOUT ANY WARRANTY; without even
+//  the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
+//  PURPOSE.  See the above copyright notice for more information.
+//============================================================================
+#ifndef vtk_m_std_void_t_h
+#define vtk_m_std_void_t_h
+
+#include <vtkm/internal/Configure.h>
+
+namespace vtkmstd
+{
+
+/// Implementation of std::void_t (C++17):
+/// Allows for specialization of class templates based on members of template
+/// parameters.
+#if defined(VTKM_GCC) && (__GNUC__ < 5)
+// Due to a defect in the wording (CWG 1558) unused parameters in alias templates
+// were not guaranteed to ensure SFINAE, and therefore would consider everything
+// to match the 'true' side. For VTK-m the only known compiler that implemented
+// this defect is GCC < 5.
+template <class... T>
+struct void_pack
+{
+  using type = void;
+};
+template <class... T>
+using void_t = typename void_pack<T...>::type;
+#else
+template <typename...>
+using void_t = void;
+#endif
+
+} // end namespace vtkmstd
+
+#endif //vtk_m_std_void_t_h