Avoid heap-allocation of ParticleDataTable

- prefer `optional` over `unique_ptr` to create a 'nullable' object
  avoiding the explicit heap allocation for ParticleDataTable

PartPropSvc/src/PartPropSvc.cpp

@@ -136,7 +136,7 @@ StatusCode PartPropSvc::createTable()
     m_upid_local = true;
   }
 
-  m_pdt.reset( new HepPDT::ParticleDataTable( m_upid_name, m_upid ) );
+  m_pdt.emplace( m_upid_name, m_upid );
 
   HepPDT::TableBuilder tb( *m_pdt );
 
@@ -166,11 +166,11 @@ HepPDT::ParticleDataTable* PartPropSvc::PDT()
     debug() << "creating ParticleDataTable" << endmsg;
     if ( createTable().isFailure() ) {
       fatal() << "Could not create ParticleDataTable" << endmsg;
-      m_pdt.reset( nullptr );
+      m_pdt.reset();
     }
   }
 
-  return m_pdt.get();
+  return m_pdt.get_ptr();
 }
 
 //* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *//

PartPropSvc/src/PartPropSvc.h

@@ -6,8 +6,9 @@
 #include "HepPDT/ParticleDataTable.hh"
 #include "HepPDT/TableBuilder.hh"
 
+#include "boost/optional.hpp"
+
 #include <iostream>
-#include <memory>
 #include <string>
 #include <utility>
 #include <vector>
@@ -38,9 +39,6 @@ public:
 
   void setUnknownParticleHandler( HepPDT::ProcessUnknownID*, const std::string& ) override;
 
-  // Destructor.
-  ~PartPropSvc() override = default;
-
 private:
   using inputFunPtr = bool ( * )( std::istream&, HepPDT::TableBuilder& );
 
@@ -52,7 +50,7 @@ private:
   HepPDT::ProcessUnknownID* m_upid = nullptr;
   std::string m_upid_name;
 
-  std::unique_ptr<HepPDT::ParticleDataTable> m_pdt;
+  boost::optional<HepPDT::ParticleDataTable> m_pdt;
 
   inputFunPtr parseTableType( const std::string& );