Just out of curiosity I removed all default parameters for the following functions:

• mitkLabelSetImage::MergeLabel
• mitk::LabelSetImage::MergeLabels
• mitk::LabelSetImage::UpdateCenterOfMass
• mitk::LabelSetImage::RemoveLabels
• mitk::LabelSetImage::EraseLabel
• mitk::LabelSetImage::EraseLabels
• mitk::LabelSetImage::GetActiveLabel
• const mitk::LabelSetImage::GetActiveLabel
• mitk::LabelSetImage::GetLabel
• mitk::LabelSetImage::GetLabelSet
• const mitk::LabelSetImage::GetLabelSet
• mitk::LabelSetImage::GetNumberOfLabels
• mitk::LabelSetImage::CreateLabelMask

And got the following errors:

I just stumbled upon the following:

Inside of the LabelSetImagethere are two functions (four with const):

• mitk::Label *GetActiveLabel(unsigned int layer = 0);
• const mitk::Label* GetActiveLabel(unsigned int layer = 0) const;

and

• mitk::LabelSet *GetActiveLabelSet();
• const mitk::LabelSet* GetActiveLabelSet() const;

Inside the GetActiveLabelSet-functions to access the vector GetActiveLayer is always used. This makes sense as the name of the function suggests.
Inside the GetActivelabel-functions to access the vector the given (possibly default) argument is used. This does not make sense and is counterintuitive. This is a special case where the default layer parameter is not only unnecessary but completely misleading. I have never seen a call to GetActiveLabel without the typical argument (GetActiveLabel(GetActiveLayer()), so removing the default parameter here will probably do no harm but remove obscurity. Since it was said to not work too much on the LabelSetImage, I just ping @floca and @kislinsk here. At least this should be considered for a new data structure, but it could also be fixed quickly.

in the new data structure it won't happen as there is no active label anymore. But you are right I thought the same when I looked through the class. My motiviation to refine this obsolete class was only limited 🙈 .