[ntuple] detect runtime member streamers via TRealData by JasMehta08 · Pull Request #21281 · root-project/root

JasMehta08 · 2026-02-14T14:43:09Z

This Pull request:

Changes or fixes:

When a class member has a custom streamer set at runtime via TClass::SetMemberStreamer() or TClass::AdoptMemberStreamer(),

RFieldBase::Create() now detects this by iterating TRealData and checking TRealData::GetStreamer().

If any member has a custom streamer, an RStreamerField is used instead of RClassField, preventing the class from being split into columns which would bypass the member's custom serialization logic.

Checklist:

tested changes locally

This PR fixes #20448

jblomer

Thank you!

I think that automatically using a streamer field is undesired. Custom member streamers are a rare case and for now, I think we should simply detect this case in RClassField and fail with an error message.

We would also need a test for this.

JasMehta08 · 2026-03-04T08:43:27Z

@jblomer

I have updated the PR,

I moved the check to RFieldMeta.cxx to detect if any member has
custom streamer present, and to throw error if it is present.
I have also added a test for the same.

Please let me know if I should change anything.

Thanks!

jblomer

Looks quite close to being merge ready!

jblomer · 2026-03-04T16:46:29Z

tree/ntuple/src/RFieldMeta.cxx

+   // Detect custom streamers set on individual members at runtime via
+   // TClass::SetMemberStreamer() or TClass::AdoptMemberStreamer().
+   // These are stored in TRealData and are not caught by CanSplit().
+   if (auto realDataList = fClass->GetListOfRealData()) {


Use the following snippet to iterate the real data:
for (auto realMember : ROOT::Detail::TRangeStaticCast<TRealData>(*fClass->GetListOfRealData())) {

jblomer · 2026-03-04T16:47:44Z

tree/ntuple/test/StreamerField.hxx

@@ -101,4 +101,11 @@ struct TemperatureKelvin {
   float fValue;
 };

+// Test class for runtime member streamers set via TClass::SetMemberStreamer
+struct MemberStreamerContainer {


Suggested change

struct MemberStreamerContainer {

struct MemberWithCustomStreamer {

jblomer · 2026-03-04T16:47:54Z

tree/ntuple/test/StreamerField.hxx

@@ -101,4 +101,11 @@ struct TemperatureKelvin {
   float fValue;
 };

+// Test class for runtime member streamers set via TClass::SetMemberStreamer
+struct MemberStreamerContainer {


This class should go in CustomStruct.hxx

jblomer · 2026-03-04T16:48:05Z

tree/ntuple/test/StreamerFieldLinkDef.h

@@ -27,4 +27,6 @@
 #pragma link C++ options = rntupleStreamerMode(true) class TemperatureCelsius + ;
 #pragma link C++ options = rntupleStreamerMode(true) class TemperatureKelvin + ;

+#pragma link C++ class MemberStreamerContainer + ;


Suggested change

#pragma link C++ class MemberStreamerContainer + ;

#pragma link C++ class MemberStreamerContainer+;

jblomer · 2026-03-04T16:49:26Z

tree/ntuple/test/rfield_streamer.cxx

+         b << *val;
+      }
+   });
+


Perhaps also check that "CanSplit" is still true

jblomer · 2026-03-04T16:49:45Z

tree/ntuple/test/rfield_streamer.cxx

@@ -407,3 +407,26 @@ TEST(RField, StreamerClassMismatch)
                         false /* matchFullMessage */);
   reader->LoadEntry(0);
 }
+
+// Detect custom streamers set on class members at runtime via TClass::SetMemberStreamer.
+TEST(RField, MemberCustomStreamer)


This test should go in rfield_class.cxx

JasMehta08 · 2026-03-04T18:49:34Z

@jblomer

i have updated the code,

i used the snippet provided by you.
also added the Cansplit() test
and move the test to rfield_class.cxx

Thanks!

tree/ntuple/test/rfield_class.cxx

github-actions · 2026-03-05T07:59:06Z

Test Results

21 files 21 suites 3d 3h 50m 29s ⏱️
3 831 tests 3 822 ✅ 1 💤 8 ❌
72 929 runs 72 824 ✅ 9 💤 96 ❌

For more details on these failures, see this check.

Results for commit f6314db.

jblomer

In principle looks good to me but see the CI report (missing TBuffer.h include)

jblomer · 2026-03-05T08:03:30Z

tree/ntuple/test/CustomStruct.hxx

+struct MemberWithCustomStreamer {
+   int fNormal = 0;
+   int fCustom = 0;
+   ClassDefNV(MemberWithCustomStreamer, 1);


Nit:

Suggested change

ClassDefNV(MemberWithCustomStreamer, 1);

ClassDefNV(MemberWithCustomStreamer, 2);

JasMehta08 · 2026-03-05T08:40:49Z

@jblomer
I think the segmentation errors is caused by the GetListOfRealData() returns nullptr because we are not checking for that and trying to access it.
Do you think that fixing that would fix the errors? if so, should I add a if check?

JasMehta08 · 2026-03-05T12:23:42Z

@jblomer

I have added the changes that you had pointed out about
the missing header and the updated the class version.

I also changed the code snippet as i think it is the cause of the segmentation fault.
Please let me know if i have gotten the reasoning behind the segmentation fault wrong.

Thanks!

JasMehta08 requested a review from jblomer as a code owner February 14, 2026 14:43

couet assigned jblomer Feb 16, 2026

jblomer requested changes Feb 27, 2026

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JasMehta08 force-pushed the detect-custom-streamers branch from c4e0f67 to 695bd21 Compare March 4, 2026 08:34

JasMehta08 requested a review from jblomer March 4, 2026 08:43

jblomer requested changes Mar 4, 2026

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JasMehta08 force-pushed the detect-custom-streamers branch from 695bd21 to b1324a7 Compare March 4, 2026 18:46

pcanal reviewed Mar 4, 2026

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tree/ntuple/test/rfield_class.cxx Outdated Show resolved Hide resolved

JasMehta08 force-pushed the detect-custom-streamers branch from b1324a7 to f6314db Compare March 5, 2026 05:12

JasMehta08 requested a review from jblomer March 5, 2026 05:13

jblomer reviewed Mar 5, 2026

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[ntuple] detect custom member streamers via TRealData

14a18ab

JasMehta08 force-pushed the detect-custom-streamers branch from f6314db to 14a18ab Compare March 5, 2026 12:10

JasMehta08 requested a review from jblomer March 5, 2026 12:41

	struct MemberStreamerContainer {
	struct MemberWithCustomStreamer {

	#pragma link C++ class MemberStreamerContainer + ;
	#pragma link C++ class MemberStreamerContainer+;

	ClassDefNV(MemberWithCustomStreamer, 1);
	ClassDefNV(MemberWithCustomStreamer, 2);

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JasMehta08 commented Feb 14, 2026

This Pull request:

Changes or fixes:

Checklist:

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JasMehta08 commented Mar 4, 2026 •

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