vtkAffineImplicitBackend: move implementation into its own file

This hides it from those just looking for the declaration and
accidentally instantiating it.

Common/Core/CMakeLists.txt

@@ -401,6 +401,7 @@ set(templates
   ${vtk_smp_templates})
 
 set(private_templates
+  vtkAffineImplicitBackend.txx
   vtkDataArrayPrivate.txx)
 
 set(vtk_include_dirs)

Common/Core/vtkAffineImplicitBackend.h

@@ -40,11 +40,7 @@ struct VTKCOMMONCORE_EXPORT vtkAffineImplicitBackend final
    * \param slope the slope of the affine function
    * \param intercept the intercept value at the origin (i.e. the value at 0)
    */
-  vtkAffineImplicitBackend(ValueType slope, ValueType intercept)
-    : Slope(slope)
-    , Intercept(intercept)
-  {
-  }
+  vtkAffineImplicitBackend(ValueType slope, ValueType intercept);
 
   /**
    * The main call method for the backend
@@ -52,10 +48,7 @@ struct VTKCOMMONCORE_EXPORT vtkAffineImplicitBackend final
    * \param index the index at which one wished to evaluate the backend
    * \return the affinely computed value
    */
-  ValueType operator()(int index) const
-  {
-    return this->Slope * static_cast<ValueType>(index) + this->Intercept;
-  }
+  ValueType operator()(int index) const;
 
   /**
    * The slope of the affine function on the indeces
@@ -69,10 +62,3 @@ struct VTKCOMMONCORE_EXPORT vtkAffineImplicitBackend final
 VTK_ABI_NAMESPACE_END
 
 #endif // vtkAffineImplicitBackend_h
-
-#ifdef VTK_AFFINE_BACKEND_INSTANTIATING
-#define VTK_INSTANTIATE_AFFINE_BACKEND(ValueType)                                                  \
-  VTK_ABI_NAMESPACE_BEGIN                                                                          \
-  template struct VTKCOMMONCORE_EXPORT vtkAffineImplicitBackend<ValueType>;                        \
-  VTK_ABI_NAMESPACE_END
-#endif

Common/Core/vtkAffineImplicitBackend.txx

+// SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
+// SPDX-License-Identifier: BSD-3-Clause
+// Funded by CEA, DAM, DIF, F-91297 Arpajon, France
+#ifndef vtkAffineImplicitBackend_txx
+#define vtkAffineImplicitBackend_txx
+
+#include "vtkAffineImplicitBackend.h"
+
+/**
+ * \struct vtkAffineImplicitBackend
+ * \brief A utility structure serving as a backend for affine (as a function of the index) implicit
+ * arrays
+ *
+ * This structure can be classified as a closure and can be called using syntax similar to a
+ * function call.
+ *
+ * At construction it takes two parameters: the slope of the map and the intercept. It returns a
+ * value calculated as:
+ *
+ *   value = slope * index + intercept
+ *
+ * An example of potential usage in a vtkImplicitArray
+ * ```
+ * double slope = some_number;
+ * double intercept = some_other_number;
+ * vtkNew<vtkImplicitArray<vtkAffineImplicitBackend<double>>> affineArray;
+ * affineArray->SetBackend(std::make_shared<vtkAffineImplicitBackend<double>>(slope, intercept));
+ * affineArray->SetNumberOfTuples(however_many_you_want);
+ * affineArray->SetNumberOfComponents(whatever_youd_like);
+ * double value = affineArray->GetTypedComponent(index_in_tuple_range, index_in_component_range);
+ * ```
+ */
+VTK_ABI_NAMESPACE_BEGIN
+template <typename ValueType>
+vtkAffineImplicitBackend<ValueType>::vtkAffineImplicitBackend(ValueType slope, ValueType intercept)
+  : Slope(slope)
+  , Intercept(intercept)
+{
+}
+
+template <typename ValueType>
+ValueType vtkAffineImplicitBackend<ValueType>::operator()(int index) const
+{
+  return this->Slope * static_cast<ValueType>(index) + this->Intercept;
+}
+
+#endif // vtkAffineImplicitBackend_txx
+
+#ifdef VTK_AFFINE_BACKEND_INSTANTIATING
+#define VTK_INSTANTIATE_AFFINE_BACKEND(ValueType)                                                  \
+  VTK_ABI_NAMESPACE_BEGIN                                                                          \
+  template struct VTKCOMMONCORE_EXPORT vtkAffineImplicitBackend<ValueType>;                        \
+  VTK_ABI_NAMESPACE_END
+#endif

Common/Core/vtkAffineImplicitBackendInstantiate.cxx.in

-/*=========================================================================
+// SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
+// SPDX-License-Identifier: BSD-3-Clause
 
-  Program:   Visualization Toolkit
-  Module:    vtkAffineImplicitBackendInstantiate_@INSTANTIATION_VALUE_TYPE@.cxx
-
-  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
-  All rights reserved.
-  See Copyright.txt or http://www.kitware.com/Copyright.htm 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.
-
-=========================================================================*/
 #define VTK_AFFINE_BACKEND_INSTANTIATING
 #include "vtkAffineImplicitBackend.h"
+#include "vtkAffineImplicitBackend.txx"
 
 VTK_INSTANTIATE_AFFINE_BACKEND(@INSTANTIATION_VALUE_TYPE@)