STYLE: Prefer c++11 [[maybe_unused]] attribute over (void)varname

The use of (void)varname was a mechanism to silence compiler
warnings prior to universal language support in c++11 for
the [[maybe_unused]] attribute specifier.

Modules/Core/Common/CMake/itkCheckHasFenvtStructMember.cxx

@@ -21,15 +21,14 @@
 int
 main()
 {
-  fenv_t fenv;
+  [[maybe_unused]] fenv_t fenv;
 #if defined(ITK_CHECK_FENV_T_CONTROL)
-  (void)sizeof(fenv.__control);
+  [[maybe_unused]] const auto tempSize = sizeof(fenv.__control);
 #elif defined(ITK_CHECK_FENV_T_CONTROL_WORD)
-  (void)sizeof(fenv.__control_word);
+  [[maybe_unused]] const auto tempSize = sizeof(fenv.__control_word);
 #elif defined(ITK_CHECK_FENV_T_CW)
-  (void)sizeof(fenv.__cw);
+  [[maybe_unused]] const auto tempSize = sizeof(fenv.__cw);
 #else
-  (void)fenv;
 #  error \
     "Unknown fenv_t struct member test: Make sure to specify a compile definition of the form -DITK_CHECK_FENV_T_xxx"
 #endif

Modules/Core/Common/include/itkObjectFactoryBase.h

@@ -218,12 +218,10 @@ class ITKCommon_EXPORT ObjectFactoryBase : public Object
     {};
 
     // Factory registration, made thread-safe by "magic statics" (as introduced with C++11).
-    static const FactoryRegistration staticFactoryRegistration = [] {
+    [[maybe_unused]] static const FactoryRegistration staticFactoryRegistration = [] {
       RegisterFactoryInternal(TFactory::New());
       return FactoryRegistration{};
     }();
-
-    (void)staticFactoryRegistration;
   }
 
   /** Initialize the static members of ObjectFactoryBase. */

Modules/Core/Common/include/itkSingletonMacro.h

@@ -25,11 +25,10 @@
 #ifndef itkSingletonMacro_h
 #define itkSingletonMacro_h
 
-#define itkInitGlobalsMacro(VarName)                       \
-  {                                                        \
-    static auto * staticGlobals = Get##VarName##Pointer(); \
-    (void)staticGlobals;                                   \
-  }                                                        \
+#define itkInitGlobalsMacro(VarName)                                        \
+  {                                                                         \
+    [[maybe_unused]] static auto * staticGlobals = Get##VarName##Pointer(); \
+  }                                                                         \
   ITK_MACROEND_NOOP_STATEMENT
 
 #define itkGetGlobalDeclarationMacro(Type, VarName) static Type * Get##VarName##Pointer()

Modules/Core/Common/include/itkVariableLengthVector.h

@@ -152,10 +152,8 @@ class ITK_TEMPLATE_EXPORT VariableLengthVector
   struct NeverReallocate : AllocateRootPolicy
   {
     bool
-    operator()(unsigned int newSize, unsigned int oldSize) const
+    operator()([[maybe_unused]] unsigned int newSize, [[maybe_unused]] unsigned int oldSize) const
     {
-      (void)newSize;
-      (void)oldSize;
       itkAssertInDebugAndIgnoreInReleaseMacro(newSize == oldSize &&
                                               "SetSize is expected to never change the VariableLengthVector size...");
       return true;

Modules/Core/Common/src/itkFloatingPointExceptions.cxx

@@ -113,7 +113,7 @@ itkFloatingPointExceptionsAbortOrExit()
   }
 }
 
-void
+[[maybe_unused]] void
 itkFloatingPointExceptionsNotSupported()
 {
   std::cerr << "FloatingPointExceptions are not supported on this platform." << std::endl;

Modules/Core/Common/src/itkFloatingPointExceptions_unix.cxx

@@ -165,7 +165,6 @@ FloatingPointExceptions::Enable()
   sigaction(SIGFPE, &act, nullptr);
 #  endif
   FloatingPointExceptions::m_PimplGlobals->m_Enabled = true;
-  (void)itkFloatingPointExceptionsNotSupported; // avoid unused-function warning
 #else
   itkFloatingPointExceptionsNotSupported();
 #endif

Modules/Core/Common/test/itkBitCastGTest.cxx

@@ -40,7 +40,6 @@ TEST(BitCast, ResultIsBitwiseEqualToArgument)
     Expect_return_value_is_bitwise_equal_to_function_argument<unsigned int>(i);
   }
 
-  int value;
-  (void)value;
+  [[maybe_unused]] int value;
   Expect_return_value_is_bitwise_equal_to_function_argument<intptr_t>(&value);
 }

Modules/Core/QuadEdgeMesh/test/itkQuadEdgeMeshEulerOperatorCreateCenterVertexTest.cxx

@@ -74,7 +74,7 @@ itkQuadEdgeMeshEulerOperatorCreateCenterVertexTest(int, char *[])
   std::cout << "OK" << std::endl;
 #endif
 
-  (void)createCenterVertex->GetNameOfClass();
+  [[maybe_unused]] const std::string name_of_class = createCenterVertex->GetNameOfClass();
 
   createCenterVertex->SetInput(mesh);
 #ifndef NDEBUG

Modules/IO/MINC/src/itkMINCImageIO.cxx

@@ -34,8 +34,6 @@ extern "C"
   void
   MINCIOFreeTmpDimHandle(unsigned int size, midimhandle_t * ptr)
   {
-    int          error = 0;
-    unsigned int x = 0;
     if (!ptr)
     {
       /*
@@ -46,16 +44,14 @@ extern "C"
 #endif
       return;
     }
-    for (; x < size; ++x)
+    for (unsigned int x = 0; x < size; ++x)
     {
-      error = mifree_dimension_handle(ptr[x]);
+      [[maybe_unused]] int error = mifree_dimension_handle(ptr[x]);
 #ifndef NDEBUG
       if (error != MI_NOERROR)
       {
         printf("MINCIOFreeTmpDimHandle: mifree_dimension_handle(ptr[%u]) returned %d", x, error);
       }
-#else
-      (void)error;
 #endif
     }
   }

Modules/IO/TransformFactory/src/itkTransformFactoryBase.cxx

@@ -73,7 +73,7 @@ TransformFactoryBase::RegisterDefaultTransforms()
   // make sure that the factory instance has
   // been created. All normal paths to this method
   // already do this but this makes certain sure it's done
-  (void)TransformFactoryBase::GetFactory();
+  [[maybe_unused]] auto currentFactory = TransformFactoryBase::GetFactory();
 
   if (!TransformFactoryBasePrivate::DefaultTransformsRegistered)
   {

Modules/Numerics/Statistics/include/itkCovarianceSampleFilter.hxx

@@ -72,8 +72,7 @@ CovarianceSampleFilter<TSample>::MakeOutput(DataObjectPointerArraySizeType index
 
   if (index == 1)
   {
-    MeasurementVectorRealType mean;
-    (void)mean; // for complainty pants : valgrind
+    [[maybe_unused]] MeasurementVectorRealType mean;
     NumericTraits<MeasurementVectorRealType>::SetLength(mean, this->GetMeasurementVectorSize());
     // NumericTraits::SetLength also initializes array to zero
     auto decoratedMean = MeasurementVectorDecoratedType::New();

Modules/Numerics/Statistics/include/itkMeanSampleFilter.hxx

@@ -54,8 +54,7 @@ template <typename TSample>
 auto
 MeanSampleFilter<TSample>::MakeOutput(DataObjectPointerArraySizeType itkNotUsed(idx)) -> DataObjectPointer
 {
-  MeasurementVectorRealType mean;
-  (void)mean; // for complainty pants : valgrind
+  [[maybe_unused]] MeasurementVectorRealType mean;
   NumericTraits<MeasurementVectorRealType>::SetLength(mean, this->GetMeasurementVectorSize());
   // NumericTraits::SetLength also initializes array to zero
   auto decoratedMean = MeasurementVectorDecoratedType::New();