STYLE: Replace std::unique_lock with std::lock_guard in Filter.hxx files

Removed explicit `unlock()` calls. Instead, just made sure that the `lock_guard`
variables go out of scope as soon as other threads are allowed to merge data.

Follow-up to pull request #4168
commit 1094cdd
"STYLE: Replace `std::unique_lock` with `std::lock_guard` in ThreadPool"

Modules/Filtering/ImageStatistics/include/itkLabelOverlapMeasuresImageFilter.hxx

@@ -120,28 +120,23 @@ LabelOverlapMeasuresImageFilter<TLabelImage>::ThreadedStreamedGenerateData(const
   // local copy, this thread may do multiple merges.
   while (true)
   {
-
+    MapType tomerge{};
     {
-      std::unique_lock<std::mutex> lock(m_Mutex);
+      const std::lock_guard<std::mutex> lockGuard(m_Mutex);
 
       if (m_LabelSetMeasures.empty())
       {
         swap(m_LabelSetMeasures, localStatistics);
         break;
       }
-      else
-      {
-        // copy the output map to thread local storage
-        MapType tomerge;
-        swap(m_LabelSetMeasures, tomerge);
 
-        // allow other threads to merge data
-        lock.unlock();
+      // Move the data of the output map to the local `tomerge` and clear the output map.
+      swap(m_LabelSetMeasures, tomerge);
 
-        // Merge tomerge into localStatistics, locally
-        MergeMap(localStatistics, tomerge);
-      }
-    } // release lock
+    } // release lock, allow other threads to merge data
+
+    // Merge tomerge into localStatistics, locally
+    MergeMap(localStatistics, tomerge);
   }
 }
 

Modules/Filtering/ImageStatistics/include/itkLabelStatisticsImageFilter.hxx

@@ -257,28 +257,23 @@ LabelStatisticsImageFilter<TInputImage, TLabelImage>::ThreadedStreamedGenerateDa
   // local copy, this thread may do multiple merges.
   while (true)
   {
-
+    MapType tomerge{};
     {
-      std::unique_lock<std::mutex> lock(m_Mutex);
+      const std::lock_guard<std::mutex> lockGuard(m_Mutex);
 
       if (m_LabelStatistics.empty())
       {
         swap(m_LabelStatistics, localStatistics);
         break;
       }
-      else
-      {
-        // copy the output map to thread local storage
-        MapType tomerge;
-        swap(m_LabelStatistics, tomerge);
 
-        // allow other threads to merge data
-        lock.unlock();
+      // Move the data of the output map to the local `tomerge` and clear the output map.
+      swap(m_LabelStatistics, tomerge);
 
-        // Merge tomerge into localStatistics, locally
-        MergeMap(localStatistics, tomerge);
-      }
-    } // release lock
+    } // release lock, allow other threads to merge data
+
+    // Merge tomerge into localStatistics, locally
+    MergeMap(localStatistics, tomerge);
   }
 }
 

Modules/Numerics/Statistics/include/itkImageToHistogramFilter.hxx

@@ -263,39 +263,35 @@ ImageToHistogramFilter<TImage>::ThreadedMergeHistogram(HistogramPointer && histo
 {
   while (true)
   {
-
-    std::unique_lock<std::mutex> lock(m_Mutex);
-
-    if (m_MergeHistogram.IsNull())
-    {
-      m_MergeHistogram = std::move(histogram);
-      return;
-    }
-    else
+    HistogramPointer tomergeHistogram{};
     {
+      const std::lock_guard<std::mutex> lockGuard(m_Mutex);
+
+      if (m_MergeHistogram.IsNull())
+      {
+        m_MergeHistogram = std::move(histogram);
+        return;
+      }
 
       // merge/reduce the local results with current values in m_MergeHistogram
 
       // take ownership locally
-      HistogramPointer tomergeHistogram;
       swap(m_MergeHistogram, tomergeHistogram);
 
-      // allow other threads to merge data
-      lock.unlock();
+    } // release lock, allow other threads to merge data
 
-      using HistogramIterator = typename HistogramType::ConstIterator;
+    using HistogramIterator = typename HistogramType::ConstIterator;
 
-      HistogramIterator hit = tomergeHistogram->Begin();
-      HistogramIterator end = tomergeHistogram->End();
+    HistogramIterator hit = tomergeHistogram->Begin();
+    HistogramIterator end = tomergeHistogram->End();
 
-      typename HistogramType::IndexType index;
+    typename HistogramType::IndexType index;
 
-      while (hit != end)
-      {
-        histogram->GetIndex(hit.GetMeasurementVector(), index);
-        histogram->IncreaseFrequencyOfIndex(index, hit.GetFrequency());
-        ++hit;
-      }
+    while (hit != end)
+    {
+      histogram->GetIndex(hit.GetMeasurementVector(), index);
+      histogram->IncreaseFrequencyOfIndex(index, hit.GetFrequency());
+      ++hit;
     }
   }
 }

Modules/Segmentation/ConnectedComponents/include/itkRelabelComponentImageFilter.hxx

@@ -96,29 +96,27 @@ RelabelComponentImageFilter<TInputImage, TOutputImage>::ParallelComputeLabels(co
   // local copy, this thread may do multiple merges.
   while (true)
   {
-    std::unique_lock<std::mutex> lock(m_Mutex);
-
-    if (m_SizeMap.empty())
-    {
-      swap(m_SizeMap, localSizeMap);
-      break;
-    }
-    else
+    MapType toMerge{};
     {
-      // copy the output map to thread local storage
-      MapType toMerge;
-      swap(m_SizeMap, toMerge);
-
-      // allow other threads to merge data
-      lock.unlock();
+      const std::lock_guard<std::mutex> lockGuard(m_Mutex);
 
-      // Merge toMerge into localSizeMap, locally
-      for (auto & sizePair : toMerge)
+      if (m_SizeMap.empty())
       {
-        localSizeMap[sizePair.first] += sizePair.second;
+        swap(m_SizeMap, localSizeMap);
+        break;
       }
+
+      // Move the data of the output map to the local `toMerge` and clear the output map.
+      swap(m_SizeMap, toMerge);
+
+    } // release lock, allow other threads to merge data
+
+    // Merge toMerge into localSizeMap, locally
+    for (auto & sizePair : toMerge)
+    {
+      localSizeMap[sizePair.first] += sizePair.second;
     }
-  } // release lock
+  }
 }