diff --git a/tree/treeplayer/inc/TTreeReaderArray.h b/tree/treeplayer/inc/TTreeReaderArray.h
index a799134bad788..9e6c9c21505a2 100644
--- a/tree/treeplayer/inc/TTreeReaderArray.h
+++ b/tree/treeplayer/inc/TTreeReaderArray.h
@@ -43,6 +43,7 @@ class TTreeReaderArrayBase : public TTreeReaderValueBase {
    std::size_t GetValueSize() const { return fImpl ? fImpl->GetValueSize(GetProxy()) : 0; }
 
 protected:
+   TTreeReaderArrayBase() = default;
    void *UntypedAt(std::size_t idx) const { return fImpl->At(GetProxy(), idx); }
    void CreateProxy() override;
    bool GetBranchAndLeaf(TBranch *&branch, TLeaf *&myLeaf, TDictionary *&branchActualType,
@@ -52,8 +53,8 @@ class TTreeReaderArrayBase : public TTreeReaderValueBase {
 
    std::unique_ptr<TVirtualCollectionReader> fImpl; // Common interface to collections
 
-   // FIXME: re-introduce once we have ClassDefInline!
-   // ClassDefOverride(TTreeReaderArrayBase, 0);//Accessor to member of an object stored in a collection
+   ClassDefInlineOverride(TTreeReaderArrayBase, 0);//Accessor to member of an object stored in a collection
+   friend class ClassDefGenerateInitInstanceLocalInjector<TTreeReaderArrayBase>;
 };
 
 class R__CLING_PTRCHECK(off) TTreeReaderUntypedArray final : public TTreeReaderArrayBase {
@@ -241,8 +242,7 @@ class R__CLING_PTRCHECK(off) TTreeReaderArray final : public ROOT::Internal::TTr
 #define R__TTreeReaderArray_TypeString(T) #T
    const char *GetDerivedTypeName() const override { return R__TTreeReaderArray_TypeString(T); }
 #undef R__TTreeReaderArray_TypeString
-   // FIXME: re-introduce once we have ClassDefTInline!
-   // ClassDefT(TTreeReaderArray, 0);//Accessor to member of an object stored in a collection
+   ClassDefInlineOverride(TTreeReaderArray, 0);//Accessor to member of an object stored in a collection
 };
 
 namespace cling {
diff --git a/tree/treeplayer/inc/TTreeReaderValue.h b/tree/treeplayer/inc/TTreeReaderValue.h
index 316672c771ef0..a4a428c23fc02 100644
--- a/tree/treeplayer/inc/TTreeReaderValue.h
+++ b/tree/treeplayer/inc/TTreeReaderValue.h
@@ -103,6 +103,7 @@ class TTreeReaderValueBase {
    virtual ~TTreeReaderValueBase();
 
 protected:
+   TTreeReaderValueBase() = default;
    TTreeReaderValueBase(TTreeReader *reader, const char *branchname, TDictionary *dict, bool opaqueRead = false);
    TTreeReaderValueBase(const TTreeReaderValueBase &);
    TTreeReaderValueBase &operator=(const TTreeReaderValueBase &);
@@ -114,7 +115,7 @@ class TTreeReaderValueBase {
    virtual void CreateProxy();
    static const char *GetBranchDataType(TBranch *branch, TDictionary *&dict, TDictionary const *curDict);
 
-   virtual const char *GetDerivedTypeName() const = 0;
+   virtual const char *GetDerivedTypeName() const { return nullptr; };
 
    Detail::TBranchProxy *GetProxy() const { return fProxy; }
 
@@ -150,9 +151,8 @@ class TTreeReaderValueBase {
     */
    bool fOpaqueRead{false};
 
-   // FIXME: re-introduce once we have ClassDefInline!
-   // ClassDefOverride(TTreeReaderValueBase, 0);//Base class for accessors to data via TTreeReader
-
+   ClassDefInline(TTreeReaderValueBase, 0);//Base class for accessors to data via TTreeReader
+   friend class ClassDefGenerateInitInstanceLocalInjector<TTreeReaderValueBase>;
    friend class ::TTreeReader;
 };
 
@@ -238,7 +238,7 @@ class R__CLING_PTRCHECK(off) TTreeReaderValue final : public ROOT::Internal::TTr
    T &operator*() { return *Get(); }
 
 protected:
-   // FIXME: use IsA() instead once we have ClassDefTInline
+   // FIXME: use IsA() instead
    /// Get the template argument as a string.
    const char *GetDerivedTypeName() const override
    {
@@ -246,8 +246,7 @@ class R__CLING_PTRCHECK(off) TTreeReaderValue final : public ROOT::Internal::TTr
       return sElementTypeName.data();
    }
 
-   // FIXME: re-introduce once we have ClassDefTInline!
-   // ClassDefT(TTreeReaderValue, 0);//Accessor to data via TTreeReader
+   ClassDefInlineOverride(TTreeReaderValue, 0);//Accessor to data via TTreeReader
 };
 
 namespace cling {