SortInvalidFirst.h

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#ifndef canvas_Persistency_Provenance_SortInvalidFirst_h
#define canvas_Persistency_Provenance_SortInvalidFirst_h

#include <functional>

namespace art {
  template <typename T>
  class SortInvalidFirst;
}

template <typename T>
class art::SortInvalidFirst {
public:
  SortInvalidFirst() = default;
  constexpr explicit SortInvalidFirst(T const& invalidValue)
    : invalidValue_{invalidValue}
  {}
  constexpr bool
  operator()(T const& left, T const& right) const
  {
    if (left == invalidValue_ && right != invalidValue_) {
      return true;
    } else if (right == invalidValue_ && left != invalidValue_) {
      return false;
    } else {
      return left < right;
    }
  }

private:
  T invalidValue_{};
};

#endif /* canvas_Persistency_Provenance_SortInvalidFirst_h */

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