STYLE: Call the empty() member function of std containers

Ran Clang-Tidy (LLVM 11.1.0) `readability-container-size-empty`, as motivated at https://clang.llvm.org/extra/clang-tidy/checks/readability-container-size-empty.html :

"The emptiness of a container should be checked using the empty() method instead of the size() method. It is not guaranteed that size() is a constant-time function, and it is generally more efficient and also shows clearer intent to use empty(). Furthermore some containers may implement the empty() method but not implement the size() method. Using empty() whenever possible makes it easier to switch to another container in the future."

Got extra motivated by having my very first microsoft/STL commit on this topic: microsoft/STL@b95ba0e "Avoid calling size() in empty() member functions" (merged from pull request microsoft/STL#1836)

Follows ITK pull request InsightSoftwareConsortium/ITK#414 commit InsightSoftwareConsortium/ITK@c82a5f2 "PERF: readability container size empty", Hans Johnson, January 2019.

Author: N-Dekker

Common/OpenCL/Filters/itkGPUBSplineBaseTransform.hxx

@@ -71,7 +71,7 @@ template <typename TScalarType, unsigned int NDimensions>
 bool
 GPUBSplineBaseTransform<TScalarType, NDimensions>::GetSourceCode(std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/Filters/itkGPUBSplineInterpolateImageFunction.hxx

@@ -75,7 +75,7 @@ template <typename TInputImage, typename TCoordRep, typename TCoefficientType>
 bool
 GPUBSplineInterpolateImageFunction<TInputImage, TCoordRep, TCoefficientType>::GetSourceCode(std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/Filters/itkGPUIdentityTransform.hxx

@@ -38,7 +38,7 @@ template <typename TScalarType, unsigned int NDimensions, typename TParentTransf
 bool
 GPUIdentityTransform<TScalarType, NDimensions, TParentTransform>::GetSourceCode(std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/Filters/itkGPULinearInterpolateImageFunction.hxx

@@ -42,7 +42,7 @@ template <typename TInputImage, typename TCoordRep>
 bool
 GPULinearInterpolateImageFunction<TInputImage, TCoordRep>::GetSourceCode(std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/Filters/itkGPUMatrixOffsetTransformBase.hxx

@@ -274,7 +274,7 @@ bool
 GPUMatrixOffsetTransformBase<TScalarType, NInputDimensions, NOutputDimensions>::GetSourceCode(
   std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/Filters/itkGPUNearestNeighborInterpolateImageFunction.hxx

@@ -42,7 +42,7 @@ template <typename TInputImage, typename TCoordRep>
 bool
 GPUNearestNeighborInterpolateImageFunction<TInputImage, TCoordRep>::GetSourceCode(std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/Filters/itkGPUResampleImageFilter.hxx

@@ -1000,7 +1000,7 @@ bool
 GPUResampleImageFilter<TInputImage, TOutputImage, TInterpolatorPrecisionType>::HasTransform(
   const GPUTransformTypeEnum type) const
 {
-  if (this->m_FilterLoopGPUKernelHandle.size() == 0)
+  if (this->m_FilterLoopGPUKernelHandle.empty())
   {
     return false;
   }
@@ -1024,7 +1024,7 @@ int
 GPUResampleImageFilter<TInputImage, TOutputImage, TInterpolatorPrecisionType>::GetTransformHandle(
   const GPUTransformTypeEnum type) const
 {
-  if (this->m_FilterLoopGPUKernelHandle.size() == 0)
+  if (this->m_FilterLoopGPUKernelHandle.empty())
   {
     return -1;
   }

Common/OpenCL/Filters/itkGPUTranslationTransformBase.hxx

@@ -181,7 +181,7 @@ template <typename TScalarType, unsigned int NDimensions>
 bool
 GPUTranslationTransformBase<TScalarType, NDimensions>::GetSourceCode(std::string & source) const
 {
-  if (this->m_Sources.size() == 0)
+  if (this->m_Sources.empty())
   {
     return false;
   }

Common/OpenCL/ITKimprovements/itkOpenCLContext.cxx

@@ -1135,7 +1135,7 @@ OpenCLContext::CreateProgramFromSourceCode(const std::string & sourceCode,
   // kernel code must exist in a text file separate from the code of the host.
   // Also the full path to the file has to be provided.
   const std::string fileName = GetOpenCLDebugFileName(oclSource);
-  if (prefixSourceCode != "")
+  if (!prefixSourceCode.empty())
   {
     std::ofstream debugfile(fileName.c_str());
     if (debugfile.is_open() == false)
@@ -1215,11 +1215,11 @@ OpenCLContext::CreateProgramFromSourceFile(const std::string & filename,
   // To work with the Intel SDK for OpenCL* - Debugger plug-in, the OpenCL*
   // kernel code must exist in a text file separate from the code of the host.
   // Also the full path to the file has to be provided.
-  if (prefixSourceCode != "")
+  if (!prefixSourceCode.empty())
   {
     fileName = GetOpenCLDebugFileName(oclSource);
   }
-  if (prefixSourceCode != "")
+  if (!prefixSourceCode.empty())
   {
     std::ofstream debugfile(fileName.c_str());
     if (debugfile.is_open() == false)

Common/OpenCL/ITKimprovements/itkOpenCLProgram.cxx

@@ -68,7 +68,7 @@ GetOpenCLMathAndOptimizationOptions(std::string & options)
 
 #ifndef OPENCL_OPTIMIZATION_OPT_DISABLE
 #  ifdef OPENCL_OPTIMIZATION_MAD_ENABLE
-  if (options.size() != 0)
+  if (!options.empty())
   {
     options.append(" ");
   }
@@ -262,7 +262,7 @@ OpenCLProgram::Build(const std::list<OpenCLDevice> & devices, const std::string
   oclOptions = !extraBuildOptions.empty() ? oclOptions + " " + extraBuildOptions : oclOptions;
 
 #if (defined(_WIN32) && defined(_DEBUG)) || !defined(NDEBUG)
-  if (GetFileName().size() > 0)
+  if (!GetFileName().empty())
   {
     const std::string message = "clBuildProgram from file '" + GetFileName() + "'";
     this->GetContext()->OpenCLDebug(message);
@@ -296,7 +296,7 @@ OpenCLProgram::Build(const std::list<OpenCLDevice> & devices, const std::string
     error = clBuildProgram(this->m_Id, devs.size(), &devs[0], oclOptions.empty() ? 0 : &oclOptions[0], 0, 0);
   }
 #else
-  if (devs.size() == 0)
+  if (devs.empty())
   {
     error = clBuildProgram(this->m_Id, 0, 0, oclOptions.empty() ? 0 : &oclOptions[0], 0, 0);
   }