[TDF] Extract logic to select list of column types for custom columns

Author: eguiraud

tree/treeplayer/inc/ROOT/TDFNodes.hxx

@@ -368,19 +368,10 @@ public:
    unsigned int GetNSlots() const { return fNSlots; }
 };
 
-template <typename F, bool PassSlotNumber = false>
-struct CustomColumnBranchTypes {
-   using type = typename CallableTraits<F>::arg_types;
-};
-
-template <typename F>
-struct CustomColumnBranchTypes<F, true> {
-   using type = RemoveFirstParameter_t<typename CallableTraits<F>::arg_types>;
-};
-
 template <typename F, bool PassSlotNumber = false>
 class TCustomColumn final : public TCustomColumnBase {
-   using BranchTypes_t = typename CustomColumnBranchTypes<F, PassSlotNumber>::type;
+   using FunParamTypes_t = typename CallableTraits<F>::arg_types;
+   using BranchTypes_t = typename TDFInternal::RemoveFirstParameterIf<PassSlotNumber, FunParamTypes_t>::type;
    using TypeInd_t = TDFInternal::GenStaticSeq_t<BranchTypes_t::list_size>;
    using ret_type = typename CallableTraits<F>::ret_type;
 

tree/treeplayer/inc/ROOT/TDFUtils.hxx

@@ -261,6 +261,17 @@ struct HistoUtils<T, true> {
    static bool HasAxisLimits(T &) { return true; }
 };
 
+/// `type` is TypeList if MustRemove is false, otherwise it is a TypeList with the first type removed
+template <bool MustRemove, typename TypeList>
+struct RemoveFirstParameterIf {
+   using type = TypeList;
+};
+
+template <typename TypeList>
+struct RemoveFirstParameterIf<true, TypeList> {
+   using type = RemoveFirstParameter_t<TypeList>;
+};
+
 } // end NS TDF
 } // end NS Internal
 } // end NS ROOT