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dpiparo merged 1 commit into from
Jan 1, 2026
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Mention the meaning of class version zero #1180

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2 changes: 1 addition & 1 deletion manual/io_custom_classes/index.md
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Expand Up @@ -207,7 +207,7 @@ For more information on TTree, see [Trees]({{ '/manual/trees' | relative_url }})

## The `ClassDef` macro

The `ClassDef` macro can be inserted in a class definition to add some reflection capabilities to it. It also attaches a "version number" to the class that can be used for [schema evolution]({{ '/manual/io/#dealing-with-changes-in-class-layouts-schema-evolution' | relative_url }}).
The `ClassDef` macro can be inserted in a class definition to add some reflection capabilities to it. It also attaches a "version number" to the class that can be used for [schema evolution]({{ '/manual/io/#dealing-with-changes-in-class-layouts-schema-evolution' | relative_url }}). A class "version number" equal to zero signals that the class cannot be written.

Having a `ClassDef` is mandatory for classes inheriting from `TObject`, otherwise it is an optional ROOT I/O performance optimization.

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