How do you find legacy internal versions of C++ classes?

[MarcoRiggirello]

Triggered by the discussion on #364 I tried to add support for the TAxis_0 and TAttAxis_0 classes that happen to be present in some of the histograms I am working with.

These internal versions however are not present in the GitHub history of the ROOT project (TAttAxis was upgraded from v3 to v4 in 2000!), thus I have no idea of what changed over time.

The only option that I got left was to make various, more or less random, attempts to make it work, as you can see on my UnROOT fork.

So, how do you usually deal with this kind of issues? How can you retrieve this kind of information?

[tamasgal]

First, let me add that errors like TAxis_0 not defined might be misleading because the way the error is reported. In my experience the streamer parsing in UnROOT is quite robust but sometimes (rarely) there is a runaway pointer which reads bits at wrong offsets. Long story short: TAxis will be correctly parsed but the version itself might be some random integer because the offsets are wrong.
I have seen such messages very often while playing around with the bootstrapping part -> the number changes when I mess around with the logic ;) Especially version 0 is quite suspicious to me but it might very well be correct as well.

Here is an example of such a version-ladder-madness #168 (comment) basically the whole issue reports adventures where you start to implement SomeClass_X and then down the line come multiple other classes with weird versions until it doesn't make sense, like the one in the linked comment: ERROR: UndefVarError: TAttAxis_8321 not defined

To answer your question how to "find legacy internal versions of C++ classes": there is no bullet proof procedure and sometimes the version is even wrong or non-existing ;) The reverse engineering process is trial and error:

• reading other sources (uproot is a great source of truth)
• ROOT sources
• a ROOT REPL and printing streamer info is of course the "real" source of truth
• dumping the raw data and inspecting/bisecting with a hex editor (ImHex is awesome!)

[MarcoRiggirello]

Especially version 0 is quite suspicious to me but it might very well be correct as well.

a ROOT REPL and printing streamer info is of course the "real" source of truth

And in fact using the ROOT REPL I can find that the version of TAxis and TAttAxis is not v0 [1]:

StreamerInfo for class: TAxis, version=10, checksum=0x5a496e70
  TNamed         BASE            offset=  0 type=67 The basis for a named object (name, title)
  TAttAxis       BASE            offset=  0 type= 0 Axis attributes     
  int            fNbins          offset=  0 type= 3 Number of bins      
  double         fXmin           offset=  0 type= 8 Low edge of first bin
  double         fXmax           offset=  0 type= 8 Upper edge of last bin
  TArrayD        fXbins          offset=  0 type=62 Bin edges array in X
  int            fFirst          offset=  0 type= 3 First bin to display
  int            fLast           offset=  0 type= 3 Last bin to display 
  unsigned short fBits2          offset=  0 type=12 Second bit status word
  bool           fTimeDisplay    offset=  0 type=18 On/off displaying time values instead of numerics
  TString        fTimeFormat     offset=  0 type=65 Date&time format, ex: 09/12/99 12:34:00
  THashList*     fLabels         offset=  0 type=64 List of labels      
  TList*         fModLabs        offset=  0 type=64 List of modified labels

StreamerInfo for class: TAttAxis, version=4, checksum=0x5c6fff3e
  int            fNdivisions     offset=  0 type= 3 Number of divisions(10000*n3 + 100*n2 + n1)
  short          fAxisColor      offset=  0 type= 2 Color of the line axis
  short          fLabelColor     offset=  0 type= 2 Color of labels     
  short          fLabelFont      offset=  0 type= 2 Font for labels     
  float          fLabelOffset    offset=  0 type= 5 Offset of labels    
  float          fLabelSize      offset=  0 type= 5 Size of labels      
  float          fTickLength     offset=  0 type= 5 Length of tick marks
  float          fTitleOffset    offset=  0 type= 5 Offset of axis title
  float          fTitleSize      offset=  0 type= 5 Size of axis title  
  short          fTitleColor     offset=  0 type= 2 Color of axis title 
  short          fTitleFont      offset=  0 type= 2 Font for axis title 

This makes me wonder what is wrong with the TH2F histograms I'm working with, but maybe that's a topic to be discussed in an issue:

julia> UnROOT.streamerfor(f, "TH2F")
UnROOT.StreamerInfo(UnROOT.TStreamerInfo{UnROOT.TObjArray}("TH2F", "", 0x689cc295, 4, UnROOT.TObjArray("", 0, Any[UnROOT.TStreamerBase
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "TH2"
  fTitle: String "2-Dim histogram base class"
  fType: Int32 0
  fSize: Int32 0
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 25310335, 0, 0, 0]
  fTypeName: String "BASE"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
  fBaseVersion: Int32 5
, UnROOT.TStreamerBase
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "TArrayF"
  fTitle: String "Array of floats"
  fType: Int32 0
  fSize: Int32 0
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 1510733553, 0, 0, 0]
  fTypeName: String "BASE"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
  fBaseVersion: Int32 1
])), Set(Any["TArrayF", "TH2"]))

julia> UnROOT.streamerfor(f, "TAxis")
UnROOT.StreamerInfo(UnROOT.TStreamerInfo{UnROOT.TObjArray}("TAxis", "", 0x5a496e70, 10, UnROOT.TObjArray("", 0, Any[UnROOT.TStreamerBase
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "TNamed"
  fTitle: String "The basis for a named object (name, title)"
  fType: Int32 67
  fSize: Int32 0
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, -541636036, 0, 0, 0]
  fTypeName: String "BASE"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
  fBaseVersion: Int32 1
, UnROOT.TStreamerBase
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "TAttAxis"
  fTitle: String "Axis attributes"
  fType: Int32 0
  fSize: Int32 0
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 1550843710, 0, 0, 0]
  fTypeName: String "BASE"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
  fBaseVersion: Int32 4
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fNbins"
  fTitle: String "Number of bins"
  fType: Int32 3
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "int"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fXmin"
  fTitle: String "Low edge of first bin"
  fType: Int32 8
  fSize: Int64 8
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "double"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fXmax"
  fTitle: String "Upper edge of last bin"
  fType: Int32 8
  fSize: Int64 8
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "double"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerObjectAny
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fXbins"
  fTitle: String "Bin edges array in X"
  fType: Int32 62
  fSize: Int32 24
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "TArrayD"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fFirst"
  fTitle: String "First bin to display"
  fType: Int32 3
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "int"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fLast"
  fTitle: String "Last bin to display"
  fType: Int32 3
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "int"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fBits2"
  fTitle: String "Second bit status word"
  fType: Int32 12
  fSize: Int64 2
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "unsigned short"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTimeDisplay"
  fTitle: String "On/off displaying time values instead of numerics"
  fType: Int32 18
  fSize: Int64 1
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "bool"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerString
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTimeFormat"
  fTitle: String "Date&time format, ex: 09/12/99 12:34:00"
  fType: Int32 65
  fSize: Int32 24
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "TString"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerObjectPointer
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fLabels"
  fTitle: String "List of labels"
  fType: Int32 64
  fSize: Int32 8
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "THashList*"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerObjectPointer
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fModLabs"
  fTitle: String "List of modified labels"
  fType: Int32 64
  fSize: Int32 8
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "TList*"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
])), Set(Any["TNamed", "TAttAxis"]))

julia> UnROOT.streamerfor(f, "TAttAxis")
UnROOT.StreamerInfo(UnROOT.TStreamerInfo{UnROOT.TObjArray}("TAttAxis", "", 0x5c6fff3e, 4, UnROOT.TObjArray("", 0, Any[UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fNdivisions"
  fTitle: String "Number of divisions(10000*n3 + 100*n2 + n1)"
  fType: Int32 3
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "int"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fAxisColor"
  fTitle: String "Color of the line axis"
  fType: Int32 2
  fSize: Int64 2
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "short"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fLabelColor"
  fTitle: String "Color of labels"
  fType: Int32 2
  fSize: Int64 2
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "short"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fLabelFont"
  fTitle: String "Font for labels"
  fType: Int32 2
  fSize: Int64 2
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "short"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fLabelOffset"
  fTitle: String "Offset of labels"
  fType: Int32 5
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "float"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fLabelSize"
  fTitle: String "Size of labels"
  fType: Int32 5
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "float"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTickLength"
  fTitle: String "Length of tick marks"
  fType: Int32 5
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "float"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTitleOffset"
  fTitle: String "Offset of axis title"
  fType: Int32 5
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "float"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTitleSize"
  fTitle: String "Size of axis title"
  fType: Int32 5
  fSize: Int64 4
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "float"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTitleColor"
  fTitle: String "Color of axis title"
  fType: Int32 2
  fSize: Int64 2
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "short"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
, UnROOT.TStreamerBasicType
  version: UInt16 0x0004
  fOffset: Int64 0
  fName: String "fTitleFont"
  fTitle: String "Font for axis title"
  fType: Int32 2
  fSize: Int64 2
  fArrayLength: Int32 0
  fArrayDim: Int32 0
  fMaxIndex: Array{Int32}((5,)) Int32[0, 0, 0, 0, 0]
  fTypeName: String "short"
  fXmin: Float64 0.0
  fXmax: Float64 0.0
  fFactor: Float64 0.0
])), Set{Any}())

[1] Isn't it worrying though that the offset is always zero?

[tamasgal]

Yes, something is wrong there. I assume that our parser for the histograms misses something in your case. Can you open an issue with that fiole?