From 7544cbd5a0302f9d53c60bf536e595288c7bafc4 Mon Sep 17 00:00:00 2001
From: Niels Dekker <N.Dekker@lumc.nl>
Date: Tue, 17 Dec 2024 18:02:01 +0100
Subject: [PATCH] ENH: Make default-constructors of RGBPixel and RGBAPixel
 `constexpr`

Tested by means of `CheckConstexprBeginAndEndOfContainer()`. Note that these
default-constructors were already `constexpr` _implicitly_, as implied by
`= default`, when `ITK_FUTURE_LEGACY_REMOVE` would be enabled.
---
 Modules/Core/Common/include/itkRGBAPixel.h     | 4 +++-
 Modules/Core/Common/include/itkRGBPixel.h      | 4 +++-
 Modules/Core/Common/test/itkRGBAPixelGTest.cxx | 5 +++++
 Modules/Core/Common/test/itkRGBPixelGTest.cxx  | 5 +++++
 4 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Modules/Core/Common/include/itkRGBAPixel.h b/Modules/Core/Common/include/itkRGBAPixel.h
index a5ceeb3bee0..ad34971a101 100644
--- a/Modules/Core/Common/include/itkRGBAPixel.h
+++ b/Modules/Core/Common/include/itkRGBAPixel.h
@@ -81,7 +81,9 @@ class ITK_TEMPLATE_EXPORT RGBAPixel : public FixedArray<TComponent, 4>
 #ifdef ITK_FUTURE_LEGACY_REMOVE
   RGBAPixel() = default;
 #else
-  RGBAPixel() { this->Fill(0); }
+  constexpr RGBAPixel()
+    : Superclass{}
+  {}
 #endif
 
   /** Pass-through constructor for the Array base class. */
diff --git a/Modules/Core/Common/include/itkRGBPixel.h b/Modules/Core/Common/include/itkRGBPixel.h
index f9a879536f9..a43f42951cc 100644
--- a/Modules/Core/Common/include/itkRGBPixel.h
+++ b/Modules/Core/Common/include/itkRGBPixel.h
@@ -80,7 +80,9 @@ class ITK_TEMPLATE_EXPORT RGBPixel : public FixedArray<TComponent, 3>
 #ifdef ITK_FUTURE_LEGACY_REMOVE
   RGBPixel() = default;
 #else
-  RGBPixel() { this->Fill(0); }
+  constexpr RGBPixel()
+    : Superclass{}
+  {}
 #endif
 
 #if defined(ITK_LEGACY_REMOVE)
diff --git a/Modules/Core/Common/test/itkRGBAPixelGTest.cxx b/Modules/Core/Common/test/itkRGBAPixelGTest.cxx
index d5760b0ffe9..16b0c285467 100644
--- a/Modules/Core/Common/test/itkRGBAPixelGTest.cxx
+++ b/Modules/Core/Common/test/itkRGBAPixelGTest.cxx
@@ -22,6 +22,11 @@
 #include <gtest/gtest.h>
 
 
+static_assert(itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBAPixel<>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBAPixel<std::uint8_t>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBAPixel<float>>());
+
+
 // Tests that a RGBAPixel that is "value-initialized" (by empty braces, `{}`) is zero-filled.
 TEST(RGBAPixel, ValueInitializedIsZeroFilled)
 {
diff --git a/Modules/Core/Common/test/itkRGBPixelGTest.cxx b/Modules/Core/Common/test/itkRGBPixelGTest.cxx
index 64a8edb4e0d..4e65888c490 100644
--- a/Modules/Core/Common/test/itkRGBPixelGTest.cxx
+++ b/Modules/Core/Common/test/itkRGBPixelGTest.cxx
@@ -22,6 +22,11 @@
 #include <gtest/gtest.h>
 
 
+static_assert(itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBPixel<>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBPixel<std::uint8_t>>() &&
+              itk::RangeGTestUtilities::CheckConstexprBeginAndEndOfContainer<itk::RGBPixel<float>>());
+
+
 // Tests that a RGBPixel that is "value-initialized" (by empty braces, `{}`) is zero-filled.
 TEST(RGBPixel, ValueInitializedIsZeroFilled)
 {