utilities.h

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* (c) Copyright 1998-2026 CERN for the benefit of the LHCb and ATLAS collaborations *
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* This software is distributed under the terms of the Apache version 2 licence,     *
* copied verbatim in the file "LICENSE".                                            *
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#pragma once
 
#include <GaudiKernel/DataObjectHandle.h>
 
namespace Gaudi::Functional {
 
  namespace Traits {
 
    // traits classes used to customize Transformer and FilterPredicate
    // Define the types to to be used as baseclass, and as in- resp. output hanldes.
    // In case a type is not specified in the traits struct, a default is used.
    //
    // The defaults are:
    //
    //      using BaseClass = Gaudi::Algorithm
    //      template <typename T> using InputHandle = DataObjectHandle<T>;
    //      template <typename T> using OutputHandle = DataObjectHandle<T>;
    //
 
    // the best way to 'compose' traits is by inheriting them one-by-one...
    template <typename... Base>
    struct use_ : Base... {};
 
    // helper classes one can inherit from to specify a specific trait
    template <typename Base>
    struct BaseClass_t {
      using BaseClass = Base;
    };
 
    template <template <typename> class Handle>
    struct InputHandle_t {
      template <typename T>
      using InputHandle = Handle<T>;
    };
 
    template <template <typename> class Handle>
    struct OutputHandle_t {
      template <typename T>
      using OutputHandle = Handle<T>;
    };
 
    template <typename Data, typename View>
    struct writeViewFor {
      template <std::same_as<Data> T>
      using OutputHandle = DataObjectWriteHandle<View, Data>;
    };
 
    // add support for objects that should reside in the TES for lifetime management, but should not
    // be used explicitly and/or directly by downstream code.
    template <typename Data>
    struct WriteOpaqueFor {
      struct OpaqueView {
        OpaqueView() = default;
        template <typename T>
        OpaqueView( T const& ) {}
      };
 
      template <std::same_as<Data> T>
      using OutputHandle = DataObjectWriteHandle<OpaqueView, Data>;
    };
 
    // this uses the defaults -- and it itself is the default ;-)
    using useDefaults = use_<>;
  } // namespace Traits
} // namespace Gaudi::Functional