COMP: Remove in class init of SmartPointer of forward declaration

[SimonRit]

GCC 7.4.1 raised the following errors on Linux:
Modules/Core/Common/include/itkSmartPointer.h:214:18: error: invalid use of incomplete type ‘using ObjectType = class itk::MultiThreaderBase {aka class itk::MultiThreaderBase}’

This is a compiler bug (as discussed in
#3877 (comment)) and the same issue has been fixed for unique_ptr by 719c3ce.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.

[N-Dekker]

Thanks again @SimonRit

It is indeed quite similar to the problem you fixed with #3877 It appears to be an old GCC bug, fixed with GCC 9.2. I just tried the following minimal example at https://godbolt.org/z/bP6frqqKo

template <typename TObjectType>
class SmartPointer
{
public:
  SmartPointer() = default;

  template <typename T>
  SmartPointer(const SmartPointer<T> &);

  ~SmartPointer()
  {
    if (m_Pointer)
    {
      m_Pointer->UnRegister();
    }
  }

  TObjectType * m_Pointer{ nullptr };
};

class ForwardDeclaredClass;

class ProcessObject
{
  SmartPointer<ForwardDeclaredClass> m_Data{};
};

The existence of a templated constructor SmartPointer(const SmartPointer<T> &) appears to trigger the old GCC bug.

[N-Dekker]

/azp run ITK.Linux.Python