PoolUtils.h

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/* @(#)root/multiproc:$Id$ */
// Author: Enrico Guiraud July 2015

/*************************************************************************
 * Copyright (C) 1995-2000, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/

#ifndef ROOT_PoolUtils
#define ROOT_PoolUtils

#include "TError.h"
#include "TList.h"
#include "TObject.h"
#include <vector>


namespace ROOT {
//////////////////////////////////////////////////////////////////////////
///
/// This namespace contains pre-defined functions to be used in
/// conjuction with TExecutor::Map and TExecutor::MapReduce.
///
//////////////////////////////////////////////////////////////////////////
   namespace ExecutorUtils {
     //////////////////////////////////////////////////////////////////////////
     /// Merge collection of TObjects.
     /// This functor looks for an implementation of the Merge method
     /// (e.g. TH1F::Merge) and calls it on the objects contained in objs.
     /// If Merge is not found, a null pointer is returned.
      template <class T>
      class ReduceObjects{
        public:
        T operator()(const std::vector<T> &objs){
         static_assert(std::is_constructible<TObject *, T>::value,
                       "The argument should be a vector of pointers to TObject or derived classes");
         if(objs.size() == 0)
            return nullptr;

         if(objs.size() == 1)
            return objs[0];

         //get first object from objs
         auto obj = objs[0];
         //get merge function
         ROOT::MergeFunc_t merge = obj->IsA()->GetMerge();
         if(!merge) {
            Error("PoolUtils::ReduceObjects", "could not find merge method for the TObject\n. Aborting operation.");
            return nullptr;
         }

         //put the rest of the objs in a list
         TList mergelist;
         unsigned NObjs = objs.size();
         for(unsigned i=1; i<NObjs; ++i) //skip first object
            mergelist.Add(objs[i]);

         //call merge
         merge(obj, &mergelist, nullptr);
         mergelist.Delete();

         //return result
         return obj;
       }
     };
   }
}

// For backward compatibility
namespace PoolUtils = ROOT::ExecutorUtils;

namespace ROOT {
   namespace Internal {
      namespace ExecutorUtils {
         // The caster casts a pointer to a TObject to a specialised type F and leaves
         // unaltered the other cases.
         template <class O, class F>
         class ResultCaster {
         public:
            static O CastIfNeeded(O &&obj)
            {
               return obj;
            }
         };
         template <class F>
         class ResultCaster<TObject *, F> {
         public:
            static typename std::enable_if<std::is_pointer<F>::value, F>::type CastIfNeeded(TObject *obj)
            {
               return static_cast<F>(obj);
            }
         };
      }
   // For backward compatibility
   namespace PoolUtils = ExecutorUtils;
   }
}


#endif