STYLE: Replace declare-then-assign with initialization at declaration

Documentation/AI/compiler-cautions.md

@@ -357,6 +357,35 @@ class Foo {
 };
 ```
 
+### 6c. `-Wunused-but-set-variable` — GCC (declare-then-assign refactoring)
+
+After converting `T x; x = func();` to `const T x = func();`, GCC warns if
+`x` is never read downstream. Apple Clang and MSVC typically don't fire this.
+This is the most common warning introduced by STYLE: declare-then-assign
+refactoring commits.
+
+```cpp
+// Triggers warning — computed but never tested:
+const MatrixType inverse = matrix.GetInverse();
+
+// Fix for test code — add a meaningful assertion:
+const MatrixType inverse = matrix.GetInverse();
+if (itk::Math::NotExactlyEquals(inverse(0, 0), expected))
+{
+  std::cerr << "Problem with GetInverse()" << std::endl;
+  return EXIT_FAILURE;
+}
+
+// Fix for production code — don't capture if unneeded:
+matrix.GetInverse();  // called for side effect only
+```
+
+In test files, **always prefer adding a test assertion** over suppressing
+with `[[maybe_unused]]` or `static_cast<void>()`. Tests that compute values
+without checking them are a missed coverage opportunity.
+
+**References:** PR #6044, CDash build 11198750.
+
 ---
 
 ## 7. MSVC-Specific Warnings
@@ -537,6 +566,7 @@ When refactoring existing code, verify each item:
 - [ ] `std::vector` replacing `new[]` — guard `data()` / `operator[]` calls on potentially empty vectors
 - [ ] Lambda captures — no unused captures; `constexpr` variables don't need capture
 - [ ] Loop variables initialized at declaration
+- [ ] Declare-then-assign conversions: if `const T x = func()` is never read downstream, add a test assertion (in test code) or restructure (in production code) — GCC `-Wunused-but-set-variable` fires on captured-but-unread return values
 - [ ] `bool` not used with `|=` for exit-status accumulation — use `int`
 - [ ] Third-party header includes wrapped with appropriate `ITK_CLANG_SUPPRESS_*` macros
 - [ ] Explicit template instantiations in shared-build modules marked `ITK_TEMPLATE_EXPORT`

Modules/Core/Common/include/itkConceptChecking.h

@@ -775,9 +775,8 @@ struct HasZero
     void
     constraints()
     {
-      T a;
+      T a = T{};
 
-      a = T{};
       Detail::IgnoreUnusedVariable(a);
     }
   };

Modules/Core/Common/include/itkImage.hxx

@@ -38,10 +38,8 @@ template <typename TPixel, unsigned int VImageDimension>
 void
 Image<TPixel, VImageDimension>::Allocate(bool initializePixels)
 {
-  SizeValueType num;
-
   this->ComputeOffsetTable();
-  num = static_cast<SizeValueType>(this->GetOffsetTable()[VImageDimension]);
+  SizeValueType num = static_cast<SizeValueType>(this->GetOffsetTable()[VImageDimension]);
 
   m_Buffer->Reserve(num, initializePixels);
 }

Modules/Core/Common/include/itkPyImageFilter.hxx

@@ -151,10 +151,8 @@ PyImageFilter<TInputImage, TOutputImage>::GenerateOutputInformation()
   // make sure that the CommandCallable is in fact callable
   if (PyCallable_Check(this->m_GenerateOutputInformationCallable))
   {
-    PyObject * result;
-
     PyObject * args = PyTuple_Pack(1, this->m_Self);
-    result = PyObject_Call(this->m_GenerateOutputInformationCallable, args, (PyObject *)NULL);
+    PyObject * result = PyObject_Call(this->m_GenerateOutputInformationCallable, args, (PyObject *)NULL);
     SWIG_Py_DECREF(args);
 
     if (result)
@@ -214,10 +212,8 @@ PyImageFilter<TInputImage, TOutputImage>::GenerateInputRequestedRegion()
   // make sure that the CommandCallable is in fact callable
   if (PyCallable_Check(this->m_GenerateInputRequestedRegionCallable))
   {
-    PyObject * result;
-
     PyObject * args = PyTuple_Pack(1, this->m_Self);
-    result = PyObject_Call(this->m_GenerateInputRequestedRegionCallable, args, (PyObject *)NULL);
+    PyObject * result = PyObject_Call(this->m_GenerateInputRequestedRegionCallable, args, (PyObject *)NULL);
     SWIG_Py_DECREF(args);
 
     if (result)
@@ -249,10 +245,8 @@ PyImageFilter<TInputImage, TOutputImage>::GenerateData()
   }
   else
   {
-    PyObject * result;
-
     PyObject * args = PyTuple_Pack(1, this->m_Self);
-    result = PyObject_Call(this->m_GenerateDataCallable, args, (PyObject *)NULL);
+    PyObject * result = PyObject_Call(this->m_GenerateDataCallable, args, (PyObject *)NULL);
     SWIG_Py_DECREF(args);
 
     if (result)

Modules/Core/Common/src/itkObjectFactoryBase.cxx

@@ -316,7 +316,7 @@ ObjectFactoryBase::LoadDynamicFactories()
 static std::string
 CreateFullPath(const char * path, const char * file)
 {
-  std::string ret;
+  std::string ret = path;
 
 #  ifdef _WIN32
   const char sep = '\\';
@@ -326,7 +326,6 @@ CreateFullPath(const char * path, const char * file)
   /**
    * make sure the end of path is a separator
    */
-  ret = path;
   if (!ret.empty() && ret.back() != sep)
   {
     ret += sep;

Modules/Core/Common/test/itkAnnulusOperatorGTest.cxx

@@ -115,8 +115,7 @@ TEST(AnnulusOperator, CreateAndInspect)
 
   annulus.CreateOperator();
 
-  OperatorType::SizeType annulusSize;
-  annulusSize = annulus.GetSize();
+  OperatorType::SizeType annulusSize = annulus.GetSize();
   std::cout << ", N = " << annulusSize << ", r = " << annulus.GetInnerRadius() << ", t = " << annulus.GetThickness()
             << ", i = " << annulus.GetInteriorValue() << ", a = " << annulus.GetAnnulusValue()
             << ", e = " << annulus.GetExteriorValue() << std::endl;

Modules/Core/Common/test/itkBoundaryConditionGTest.cxx

@@ -102,14 +102,12 @@ TEST(BoundaryCondition, ConstantBoundaryAtImageEdge)
   cbc.SetConstant(0.0f);
   it2d.OverrideBoundaryCondition(&cbc);
 
-  SmartIteratorType::NeighborhoodType tempN;
-  SmartIteratorType::NeighborhoodType temp2N;
-  temp2N = it2d.GetNeighborhood(); // initialize
+  SmartIteratorType::NeighborhoodType temp2N = it2d.GetNeighborhood();
 
   it2d.GoToEnd();
   --it2d;
-  tempN = it2d.GetNeighborhood();
 
+  SmartIteratorType::NeighborhoodType tempN = it2d.GetNeighborhood();
   printn(tempN.GetBufferReference(), tempN.GetSize());
 
   // The 2D image is 30x15, filled with 100*j + i.
@@ -169,8 +167,7 @@ TEST(BoundaryCondition, ZeroFluxNeumannBoundaryTraversal)
   cbc.SetConstant(0.0f);
   it2d.OverrideBoundaryCondition(&cbc);
 
-  SmartIteratorType::NeighborhoodType temp2N;
-  temp2N = it2d.GetNeighborhood(); // initialize
+  SmartIteratorType::NeighborhoodType temp2N = it2d.GetNeighborhood();
 
   itk::ZeroFluxNeumannBoundaryCondition<ImageType2D> neumann;
   for (int yak = 0; yak < 2; ++yak)

Modules/Core/Common/test/itkConstNeighborhoodIteratorWithOnlyIndexTest.cxx

@@ -208,10 +208,9 @@ itkConstNeighborhoodIteratorWithOnlyIndexTestRun()
 
   std::cout << "Creating ConstNeighborhoodIterator" << std::endl;
   ConstNeighborhoodIteratorType ra_it(radius, ra_img, ra_img->GetRequestedRegion());
-  ConstNeighborhoodIteratorType copy_it;
+  ConstNeighborhoodIteratorType copy_it = ra_it;
 
   std::cout << "Test copying." << std::endl;
-  copy_it = ra_it;
   if (copy_it != ra_it || !(copy_it == ra_it))
   {
     std::cerr << "Failure with copying or equality comparison." << std::endl;

Modules/Core/Common/test/itkExceptionObjectTest.cxx

@@ -159,8 +159,7 @@ itkExceptionObjectTest(int, char *[])
   Se.SetLocation("SE LOCATION");
   Se.SetDescription("SE DESCRIPTION");
   itk::SampleError Sf(Se);
-  itk::SampleError Sg;
-  Sg = Sf;
+  itk::SampleError Sg = Sf;
   std::cout << Sg << std::endl;
 
 

Modules/Core/Common/test/itkExtractImageTest.cxx

@@ -133,8 +133,8 @@ itkExtractImageTest(int, char *[])
   std::cout << "Input Image: " << if2 << std::endl;
 
   // Create a filter
-  itk::ExtractImageFilter<ShortImage, ShortImage>::Pointer extract;
-  extract = itk::ExtractImageFilter<ShortImage, ShortImage>::New();
+  const itk::ExtractImageFilter<ShortImage, ShortImage>::Pointer extract =
+    itk::ExtractImageFilter<ShortImage, ShortImage>::New();
   extract->SetInput(if2);
 
   // fill in an image
@@ -218,8 +218,8 @@ itkExtractImageTest(int, char *[])
   extract->SetExtractionRegion(extractRegion);
 
   // Create a stream
-  itk::StreamingImageFilter<ShortImage, ShortImage>::Pointer stream;
-  stream = itk::StreamingImageFilter<ShortImage, ShortImage>::New();
+  const itk::StreamingImageFilter<ShortImage, ShortImage>::Pointer stream =
+    itk::StreamingImageFilter<ShortImage, ShortImage>::New();
   stream->SetInput(extract->GetOutput());
   stream->SetNumberOfStreamDivisions(2);
 
@@ -289,8 +289,8 @@ itkExtractImageTest(int, char *[])
   }
 
   // Case 3: Try extracting a single row
-  itk::ExtractImageFilter<ShortImage, LineImage>::Pointer lineExtract;
-  lineExtract = itk::ExtractImageFilter<ShortImage, LineImage>::New();
+  const itk::ExtractImageFilter<ShortImage, LineImage>::Pointer lineExtract =
+    itk::ExtractImageFilter<ShortImage, LineImage>::New();
   lineExtract->SetDirectionCollapseToGuess();
   lineExtract->SetInput(if2);
 
@@ -307,9 +307,8 @@ itkExtractImageTest(int, char *[])
   std::cout << "After 1D extraction. " << std::endl;
 
   // test the dimension collapse
-  LineImage::RegionType requestedLineRegion;
+  LineImage::RegionType requestedLineRegion = lineExtract->GetOutput()->GetRequestedRegion();
 
-  requestedLineRegion = lineExtract->GetOutput()->GetRequestedRegion();
 
   itk::ImageRegionIterator<LineImage> iteratorLineIn(lineExtract->GetOutput(), requestedLineRegion);
 

Modules/Core/Common/test/itkImageIteratorTest.cxx

@@ -96,7 +96,8 @@ itkImageIteratorTest(int, char *[])
   // Exercise copy constructor
   const VectorImageIterator itr3(itr1);
 
-  // Exercise assignment operator
+  // Exercise assignment operator — intentionally two-line; do NOT merge
+  // into `VectorImageIterator itr4 = itr1` which invokes copy constructor.
   VectorImageIterator itr4;
   itr4 = itr1;
 

Modules/Core/Common/test/itkImageLinearIteratorTest.cxx

@@ -231,8 +231,7 @@ itkImageLinearIteratorTest(int, char *[])
     bot.SetDirection(0); // 0=x, 1=y, 2=z
     bot.GoToBegin();
 
-    ImageType::IndexType testIndex;
-    testIndex = start;
+    ImageType::IndexType testIndex = start;
     testIndex[1] += 2; // advance two lines in Y
 
     bot.NextLine(); // advance two lines in Y
@@ -274,8 +273,7 @@ itkImageLinearIteratorTest(int, char *[])
     cbot.SetDirection(0); // 0=x, 1=y, 2=z
     cbot.GoToBegin();
 
-    ImageType::IndexType testIndex;
-    testIndex = start;
+    ImageType::IndexType testIndex = start;
     testIndex[1] += 2; // advance two lines in Y
 
     cbot.NextLine(); // advance two lines in Y

Modules/Core/Common/test/itkImageRegionConstIteratorWithOnlyIndexTest.cxx

@@ -122,9 +122,8 @@ class itkImageRegionConstIteratorWithOnlyIndexTestIteratorTester
     }
 
     // Test iterating fwd by line
-    IndexType index;
     it.GoToBegin();
-    index = it.GetIndex();
+    IndexType index = it.GetIndex();
     for (unsigned int i = 0; i < region.GetSize()[0]; ++i)
     {
       ++it;

Modules/Core/Common/test/itkMatrixTest.cxx

@@ -76,14 +76,34 @@ itkMatrixTest(int, char *[])
   matrix2.SetIdentity();
   matrix2.GetVnlMatrix()(0, 0) = 10;
 
-  MatrixType matrixProduct;
-  matrixProduct = matrix * matrix2;
+  MatrixType matrixProduct = matrix * matrix2;
 
-  MatrixType matrix3;
-  matrix3 = matrix.GetInverse();
+  // identity * matrix2 should equal matrix2
+  if (itk::Math::NotExactlyEquals(matrixProduct(0, 0), 10) || itk::Math::NotExactlyEquals(matrixProduct(1, 1), 1) ||
+      itk::Math::NotExactlyEquals(matrixProduct(2, 2), 1) || itk::Math::NotExactlyEquals(matrixProduct(0, 1), 0) ||
+      itk::Math::NotExactlyEquals(matrixProduct(1, 0), 0))
+  {
+    std::cerr << "Problem with matrix product" << std::endl;
+    return EXIT_FAILURE;
+  }
+
+  const MatrixType matrix3 = matrix.GetInverse();
+  // Inverse of identity should be identity
+  if (itk::Math::NotExactlyEquals(matrix3(0, 0), 1) || itk::Math::NotExactlyEquals(matrix3(1, 1), 1) ||
+      itk::Math::NotExactlyEquals(matrix3(2, 2), 1) || itk::Math::NotExactlyEquals(matrix3(0, 1), 0))
+  {
+    std::cerr << "Problem with GetInverse()" << std::endl;
+    return EXIT_FAILURE;
+  }
 
-  MatrixType matrix4;
-  matrix4 = matrix.GetTranspose();
+  const MatrixType matrix4 = matrix.GetTranspose();
+  // Transpose of identity should be identity
+  if (itk::Math::NotExactlyEquals(matrix4(0, 0), 1) || itk::Math::NotExactlyEquals(matrix4(1, 1), 1) ||
+      itk::Math::NotExactlyEquals(matrix4(2, 2), 1) || itk::Math::NotExactlyEquals(matrix4(0, 1), 0))
+  {
+    std::cerr << "Problem with GetTranspose()" << std::endl;
+    return EXIT_FAILURE;
+  }
 
   auto matrix5 = itk::MakeFilled<MatrixType>(1.7);
 
@@ -217,11 +237,9 @@ itkMatrixTest(int, char *[])
       }
     }
 
-    AddSubtractMatrixType m3;
-    m3 = m1 + m2;
+    AddSubtractMatrixType m3 = m1 + m2;
 
-    AddSubtractMatrixType m4;
-    m4 = m1 - m2;
+    AddSubtractMatrixType m4 = m1 - m2;
 
     std::cout << "Results of ITK matrix addition" << std::endl;
     std::cout << "M1 = " << std::endl << m1 << std::endl;
@@ -311,8 +329,7 @@ itkMatrixTest(int, char *[])
       }
     }
 
-    MatrixType matrixC;
-    matrixC = vnlMatrixA; // Test assignment
+    MatrixType matrixC = vnlMatrixA;
 
     { // verify values
       constexpr double tolerance{ 1e-7 };

Modules/Core/Common/test/itkNeighborhoodAlgorithmTest.cxx

@@ -29,9 +29,8 @@ ImageBoundaryFaceCalculatorTest(TImage *                          image,
   using FaceCalculatorType = itk::NeighborhoodAlgorithm::ImageBoundaryFacesCalculator<TImage>;
   using FaceListType = typename FaceCalculatorType::FaceListType;
   FaceCalculatorType faceCalculator;
-  FaceListType       faceList;
+  FaceListType       faceList = faceCalculator(image, region, radius);
 
-  faceList = faceCalculator(image, region, radius);
   for (auto fit = faceList.begin(); fit != faceList.end(); ++fit)
   {
     std::cout << "Number of pixels : " << fit->GetNumberOfPixels() << std::endl;

Modules/Core/Common/test/itkRangeGTestUtilities.h

@@ -82,6 +82,8 @@ class RangeGTestUtilities
   static void
   ExpectCopyAssignedRangeHasSameIteratorsAsOriginal(const TRange & originalRange)
   {
+    // Intentionally two-line; do NOT merge into `TRange copyAssignedRange = originalRange`
+    // which invokes copy constructor instead of operator=.
     TRange copyAssignedRange;
     copyAssignedRange = originalRange;
 
@@ -106,6 +108,8 @@ class RangeGTestUtilities
   {
     const TRange originalRangeBeforeMove = originalRange;
 
+    // Intentionally two-line; do NOT merge into `TRange moveAssignedRange = std::move(...)`
+    // which invokes move constructor instead of operator=.
     TRange moveAssignedRange;
     moveAssignedRange = std::move(originalRange);
 

Modules/Core/Common/test/itkSliceIteratorTest.cxx

@@ -25,13 +25,12 @@ template <typename TPixelType, unsigned int VDimension>
 void
 FillRegionSequential(itk::SmartPointer<itk::Image<TPixelType, VDimension>> I)
 {
-  itk::Size<VDimension> Index;
+  itk::Size<VDimension> Index = (I->GetRequestedRegion()).GetSize();
   unsigned long         Location[VDimension];
 
 
   itk::ImageRegionIterator<itk::Image<TPixelType, VDimension>> data(I, I->GetRequestedRegion());
 
-  Index = (I->GetRequestedRegion()).GetSize();
 
   unsigned int ArrayLength = 1;
   for (unsigned int iDim = 0; iDim < VDimension; ++iDim)