[RDF] Unify members of Snapshot helpers into structs.

In order to implement snapshots with systematic variations, it is
beneficial to place all members related to output branches into structs.
At present, this only constitutes a move from member of SnapshotHelper
to member of RBranchData struct.

The TTree snapshot helpers are changed accordingly.

Co-authored-by: Vincenzo Eduardo Padulano <[email protected]>

Author: hageboeck

tree/dataframe/inc/ROOT/RDF/SnapshotHelpers.hxx

@@ -109,6 +109,34 @@ public:
    UntypedSnapshotRNTupleHelper MakeNew(void *newName);
 };
 
+/// Stores properties of each output branch in a Snapshot.
+struct RBranchData {
+   std::string fInputBranchName; // This contains resolved aliases
+   std::string fOutputBranchName;
+   const std::type_info *fInputTypeID = nullptr;
+   TBranch *fOutputBranch = nullptr;
+   void *fBranchAddressForCArrays = nullptr; // Used to detect if branch addresses need to be updated
+
+   std::unique_ptr<void, std::function<void(void *)>> fEmptyInstance;
+   bool fIsCArray = false;
+   bool fIsDefine = false;
+
+   RBranchData(std::string inputBranchName, std::string outputBranchName, bool isDefine, const std::type_info *typeID,
+               TBranch *outputBranch = nullptr)
+      : fInputBranchName{std::move(inputBranchName)},
+        fOutputBranchName{std::move(outputBranchName)},
+        fInputTypeID{typeID},
+        fOutputBranch{outputBranch},
+        fIsDefine(isDefine)
+   {
+   }
+   void ClearBranchPointers()
+   {
+      fOutputBranch = nullptr;
+      fBranchAddressForCArrays = nullptr;
+   }
+};
+
 class R__CLING_PTRCHECK(off) UntypedSnapshotTTreeHelper final : public RActionImpl<UntypedSnapshotTTreeHelper> {
    std::string fFileName;
    std::string fDirName;
@@ -117,17 +145,10 @@ class R__CLING_PTRCHECK(off) UntypedSnapshotTTreeHelper final : public RActionIm
    std::unique_ptr<TFile> fOutputFile;
    std::unique_ptr<TTree> fOutputTree; // must be a ptr because TTrees are not copy/move constructible
    bool fBranchAddressesNeedReset{true};
-   ColumnNames_t fInputBranchNames; // This contains the resolved aliases
-   ColumnNames_t fOutputBranchNames;
    TTree *fInputTree = nullptr; // Current input tree. Set at initialization time (`InitTask`)
-   // TODO we might be able to unify fBranches, fBranchAddresses and fOutputBranches
-   std::vector<TBranch *> fBranches;     // Addresses of branches in output, non-null only for the ones holding C arrays
-   std::vector<void *> fBranchAddresses; // Addresses of objects associated to output branches
-   RBranchSet fOutputBranches;
-   std::vector<bool> fIsDefine;
+   std::vector<RBranchData> fBranchData; // Information for all output branches
    ROOT::Detail::RDF::RLoopManager *fOutputLoopManager;
    ROOT::Detail::RDF::RLoopManager *fInputLoopManager;
-   std::vector<const std::type_info *> fInputColumnTypeIDs; // Types for the input columns
 
 public:
    UntypedSnapshotTTreeHelper(std::string_view filename, std::string_view dirname, std::string_view treename,
@@ -176,11 +197,7 @@ class R__CLING_PTRCHECK(off) UntypedSnapshotTTreeHelperMT final : public RAction
    std::vector<std::unique_ptr<TTree>> fOutputTrees;
    std::vector<int> fBranchAddressesNeedReset; // vector<bool> does not allow concurrent writing of different elements
    std::vector<TTree *> fInputTrees; // Current input trees, one per slot. Set at initialization time (`InitTask`)
-   // Addresses of branches in output per slot, non-null only for the ones holding C arrays
-   std::vector<std::vector<TBranch *>> fBranches;
-   // Addresses of objects associated to output branches per slot, non-null only for the ones holding C arrays
-   std::vector<std::vector<void *>> fBranchAddresses;
-   std::vector<RBranchSet> fOutputBranches; // Unique set of output branches, one per slot.
+   std::vector<std::vector<RBranchData>> fBranchData; // Information for all output branches of each slot
 
    // Attributes of the output TTree
 
@@ -189,16 +206,11 @@ class R__CLING_PTRCHECK(off) UntypedSnapshotTTreeHelperMT final : public RAction
    std::string fTreeName;
    TFile *fOutputFile; // Non-owning view on the output file
    RSnapshotOptions fOptions;
-   std::vector<std::string> fOutputBranchNames;
 
    // Attributes related to the computation graph
 
    ROOT::Detail::RDF::RLoopManager *fOutputLoopManager;
    ROOT::Detail::RDF::RLoopManager *fInputLoopManager;
-   std::vector<std::string> fInputBranchNames;              // This contains the resolved aliases
-   std::vector<const std::type_info *> fInputColumnTypeIDs; // Types for the input columns
-
-   std::vector<bool> fIsDefine;
 
 public:
    UntypedSnapshotTTreeHelperMT(unsigned int nSlots, std::string_view filename, std::string_view dirname,