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81 lines
3.3 KiB
Markdown
81 lines
3.3 KiB
Markdown
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# `caf::PdmNestedCollection` — design notes
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## Context
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`caf::PdmNestedCollection` is the base class for tree-shaped PDM containers — a
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folder of items that can also contain folders of the same kind. The design is a
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non-template concrete base + a CRTP template:
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```cpp
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class PdmNestedCollectionBase : public PdmObject
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{
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QString collectionName() const;
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void setCollectionName( const QString& name );
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virtual bool canAddSubCollection() const;
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virtual PdmObject* addNewSubCollection();
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void setAsTopmostFolder();
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PdmField<QString> m_collectionName;
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};
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template <typename SelfT, typename ItemT>
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class PdmNestedCollection : public PdmNestedCollectionBase
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{
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// typed accessors + storage for items and subcollections
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PdmChildArrayField<SelfT*> m_subCollections;
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PdmChildArrayField<ItemT*> m_items;
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};
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```
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`PdmNestedCollectionBase` is the shared PdmObject ancestor — the anchor point
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where the CAF method factory can register generic script methods (like
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`AddFolder`) once and have them surface on every derived nested collection via
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the inheritance walk and Python class hierarchy.
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`PdmNestedCollection<SelfT, ItemT>` adds typed storage and accessors. `SelfT` is
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the CRTP self type (so subcollections are typed `SelfT*`); `ItemT` is the leaf
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item type. The template does not inherit `PdmObjectCollection<ItemT>` — m_items
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is declared here directly, so the inheritance chain stays linear and free of
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diamond shapes.
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## Why both a non-template base and a CRTP template
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Two needs that don't fit into a single layer:
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- **CAF method registration is keyed by `classKeyword`** — only a concrete
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non-template class can serve as the anchor. Hence the non-template
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`PdmNestedCollectionBase`. Without it, generic script methods (`AddFolder`)
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would have to be re-registered per derived class.
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- **Typed storage and accessors** — `m_subCollections : PdmChildArrayField<SelfT*>`
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needs `SelfT`, which only exists as a template parameter. Hence the CRTP
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template layer on top of the base.
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## Alternative considered: non-templated base only
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```cpp
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class PdmNestedCollection : public PdmObject
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{
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...
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virtual PdmObject* createSubCollection() const = 0;
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// m_subCollectionsField / m_itemsField stored as type-erased handles
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};
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```
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Each derived class would own its typed `PdmChildArrayField<DerivedType*>
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m_subCollections` and `PdmChildArrayField<ItemType*> m_items`, register them
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with the base in the constructor, and override `createSubCollection()`. About
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~25 lines of boilerplate per derived class beyond what the current design needs:
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two field declarations, two `registerXxxField` calls, an override of
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`createSubCollection`, re-declared typed accessors.
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At the projected 8–10 derived classes that's ~200–250 lines of repeated code.
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The hybrid (non-template base + template) design pays the cost once in the
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framework and gets typed access plus generic registration for free.
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## When to revisit
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- If the count of nested-collection types drops to 1–2 — the template tier no
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longer earns its keep, and a pure non-template base wins on simplicity.
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- If template instantiation cost shows up in build profiling for the PDM layer.
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- If the typed accessors are rarely used in practice and most call sites
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immediately cast to `PdmObject*` anyway.
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