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[skip-ci][ntuple] Clarify collection proxy specifications #16688

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Oct 15, 2024
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[skip-ci][ntuple] Clarify collection proxy specifications #16688

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9861475
[ntuple] Clarify collection proxy specs
enirolf Oct 15, 2024
c0ce2c4
[ntuple] Fix typo
enirolf Oct 15, 2024
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15 changes: 9 additions & 6 deletions tree/ntuple/v7/doc/specifications.md
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Expand Up @@ -942,7 +942,7 @@ While STL (unordered) (multi)sets by definition are associative containers
on disk they are represented as sequential collections.
This means that they have the same on-disk representation as `std::vector<T>`, using two fields:
- Collection parent field whose principal column is of type `(Split)Index[64|32]`.
- Child field of type `T`, which must by a type with RNTuple I/O support.
- Child field of type `T`, which must be a type with RNTuple I/O support.
The name of the child field is `_0`.

#### std::map\<K, V\>, std::unordered_map\<K, V\>, std::multimap\<K, V\>, std::unordered_multimap\<K, V\>
Expand Down Expand Up @@ -988,13 +988,16 @@ The field name of base class subfields are numbered and preceded by a colon (`:`
#### Classes with an associated collection proxy

User classes that specify a collection proxy behave as collections of a given value type.
Associative collections are not currently supported.

The on-disk representation is similar to a `std::vector<T>` where `T` is the value type;
specifically, it is stored as two fields:
The on-disk representation of non-associative collections is identical to a `std::vector<T>`, using two fields:
- Collection parent field whose principal column is of type `(Split)Index[64|32]`.
- Child field of type `T`, which must by a type with RNTuple I/O support.
The name of the child field is `_0`.
- Child field of type `T`, which must be a type with RNTuple I/O support.

The on-disk representation of associative collections is identical to a `std::map<K, V>`, using two fields:
- Collection parent field whose principal column is of type `(Split)Index[64|32]`.
- Child field of type `std::pair<K, V>`, where `K` and `V` must be types with RNTuple I/O support.

N.B., proxy-based associative collections are supported in the RNTuple binary format, but currently are not implemented in ROOT's RNTuple reader and writer. This will be added in the future.

### ROOT::Experimental::RNTupleCardinality<SizeT>

Expand Down