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

Merge variable declaration and subsequent assignment into a single
initialization-at-declaration statement across Modules/Core/Common.
Fixes forward-reference hazards in itkPyImageFilter.hxx (args used
before declared) and itkImage.hxx (offset table read before computed).
Preserves two-line form where copy/move assignment semantics are
intentionally tested. Add const to immutable variables on changed lines.

Author: hjmjohnson

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);