junrushao commented on code in PR #670:
URL: https://github.com/apache/tvm-ffi/pull/670#discussion_r3592638199
##########
include/tvm/ffi/object.h:
##########
@@ -546,6 +546,19 @@ inline constexpr bool is_qualified_object_v =
} // namespace details
+/*!
+ * \brief Mark a forward-declared type as an Object subclass.
+ *
+ * Invoke this macro at global namespace scope after forward declaring Type.
+ * Generated ABI headers use it so recursive container fields can instantiate
+ * ObjectPtr traits before the pointee type's definition is complete.
+ *
+ * \param Type The fully qualified Object subclass.
+ */
+#define TVM_FFI_DECLARE_OBJECT_SUBCLASS(Type) \
+ template <> \
+ inline constexpr bool tvm::ffi::details::is_object_subclass_v<Type> = true
Review Comment:
A minimal example of the ordering this enables is:
```cpp
namespace example {
struct NodeObj;
}
template <>
inline constexpr bool
tvm::ffi::is_object_subclass_v<::example::NodeObj> = true;
namespace example {
struct NodeObj : public tvm::ffi::Object {
tvm::ffi::List<tvm::ffi::ObjectPtr<NodeObj>> children;
};
}
```
When `children` is declared, `NodeObj` remains incomplete until the closing
brace. `List` instantiates `TypeTraits<ObjectPtr<NodeObj>>`, so without the
specialization the primary `std::is_base_of_v<Object, NodeObj>` is ill-formed.
Generated headers now follow exactly this forward-declare → specialize → define
ordering.
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