MakeDerivedVariant.h

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/*
  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
  */
#ifndef MakeDerivedVariant_h
#define MakeDerivedVariant_h
 
namespace {
namespace MakeDerivedVariant {
   template <typename T>
   static T lvalue(T &&a) { return a;}
 
   // dummy method returning a variant which allows for one additional type than the given variant
   template <class T, typename...Args>
   constexpr auto extend(const std::variant<Args...> &, const T &) -> std::variant<Args..., T> {
      std::variant<Args..., T> ret;
      return ret;
   }
 
   // helper structor to hold the declaration of the variant
   template <typename TypeHelper, typename VariantType, std::size_t N = std::variant_size_v<VariantType> >
   struct MakeVariant {
      using variant_type = decltype( extend(MakeVariant<TypeHelper, VariantType, N-1>::m_val,
                                            typename TypeHelper::template type<decltype( lvalue( std::get<N-1>(VariantType{})))> {}  ) );
      variant_type m_val;
   };
 
   // specialisation of above helper structure for N=1 i.e. the variant just allowing T<1>
   template<typename TypeHelper, typename VariantType>
   struct MakeVariant<TypeHelper, VariantType,1> {
      using variant_type = std::variant< typename TypeHelper::template type< decltype( lvalue( std::get<0>(VariantType{}))) > > ;
      variant_type m_val;
   };
 
}
}
#endif