Transient.h

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#ifndef canvas_Persistency_Provenance_Transient_h
#define canvas_Persistency_Provenance_Transient_h
// vim: set sw=2 expandtab :

// ===================================================================
//  We give all instantiations of this template a 'transient="true"'
//  attribute for the value_ data member to tell ROOT to never write
//  it to disk.  We also set all instantiations (by hand!) to have a
//  custom streamer (TransientStreamer) that makes sure if the ROOT
//  I/O buffer is reused for reading that the data member _value is
//  reinitialized with a default-constructed T.
// ===================================================================

namespace art {
  template <typename T>
  class Transient {
  public:
    using value_type = T;
    Transient(T value = {}) : value_(value) {}
    operator T() const { return value_; }
    Transient&
    operator=(T rh)
    {
      value_ = rh;
      return *this;
    }
    T const&
    get() const noexcept
    {
      return value_;
    }
    T&
    get() noexcept
    {
      return value_;
    }

  private:
    T value_;
  };
}
#endif /* canvas_Persistency_Provenance_Transient_h */

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