Producer.h

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#ifndef GAUDI_FUNCTIONAL_PRODUCER_H
#define GAUDI_FUNCTIONAL_PRODUCER_H

#include <utility>
#include "GaudiAlg/FunctionalDetails.h"
#include "GaudiAlg/FunctionalUtilities.h"

namespace Gaudi { namespace Functional {

   template <typename Signature, typename Traits_ = Traits::useDefaults> class Producer;

   template <typename... Out, typename Traits_>
   class Producer<std::tuple<Out...>(), Traits_>
   : public details::DataHandleMixin<std::tuple<Out...>,void,Traits_> {
   public:
       using details::DataHandleMixin<std::tuple<Out...>,void,Traits_>::DataHandleMixin;

       // derived classes are NOT allowed to implement execute ...
       StatusCode execute() override final
       { return invoke(std::index_sequence_for<Out...>{}); }

       // ... instead, they must implement the following operator
       virtual std::tuple<Out...> operator()() const = 0;

   private:
       template <std::size_t... O>
       StatusCode invoke(std::index_sequence<O...>) {
           using details::as_const; using details::put;
           try {
               std::initializer_list<int> {
                    (put(std::get<O>(this->m_outputs),std::get<O>( as_const(*this)() )),0)...
               };
           } catch ( GaudiException& e ) {
               (e.code() ? this->warning() : this->error() )
                   << e.message() << endmsg;
               return e.code();
           }
           return StatusCode::SUCCESS;
       }
   };

   template <typename Out, typename Traits_>
   class Producer<Out(),Traits_>
   : public details::DataHandleMixin<std::tuple<Out>,void,Traits_> {
   public:
       using details::DataHandleMixin<std::tuple<Out>,void,Traits_>::DataHandleMixin;
       // derived classes are NOT allowed to implement execute ...
       StatusCode execute() override final {
           using details::as_const; using details::put;
           try {
                put(std::get<0>(this->m_outputs),as_const(*this)() );
           } catch ( GaudiException& e ) {
               this->error() << "Error during transform: " << e.message() << " returning " << e.code() << endmsg;
               return e.code();
           }
           return StatusCode::SUCCESS;
       }

       // ... instead, they must implement the following operator
       virtual Out operator()() const = 0;
   };
}}

#endif