Struct mx_base_vtable 

#[repr(C)]
pub struct mx_base_vtable {
    pub drop: unsafe extern "C" fn(ptr: *mut mx_erased),
    pub concrete_tag: mx_tag,
    pub query: unsafe extern "C" fn(ptr: *mut mx_erased, trait_tag: mx_tag) -> *const c_void,
    pub data_offset: usize,
}

Base vtable present in all erased objects.

This vtable provides the minimal operations needed for any erased object:

• Destructor for cleanup when R garbage collects the wrapper
• Concrete type tag for type-safe downcasts
• Query function to retrieve interface vtables

Layout Guarantee

This type is #[repr(C)] and its layout is frozen. Fields will never be reordered, and new fields will only be appended at the end.

Generated By

#[derive(ExternalPtr)] emits a static instance of this vtable for each wrapped type.

Fields

drop: unsafe extern "C" fn(ptr: *mut mx_erased)

Destructor called when the R external pointer is garbage collected.

Receives a pointer to the erased object (not the data pointer). Must deallocate the entire erased wrapper structure.

Safety

• ptr must be a valid pointer to mx_erased allocated by this type’s constructor
• Must only be called once per object
• Must be called on R’s main thread (during GC finalization)

concrete_tag: mx_tag

Tag identifying the concrete type wrapped by this object.

Used for type-safe downcasts: if concrete_tag matches the expected type’s tag, the data pointer can be cast to that type.

query: unsafe extern "C" fn(ptr: *mut mx_erased, trait_tag: mx_tag) -> *const c_void

Query function to retrieve interface vtables.

Given a trait tag, returns a pointer to the vtable for that interface, or null if the type does not implement that trait.

Arguments

• ptr - Pointer to the erased object
• trait_tag - Tag identifying the requested trait interface

Returns

• Non-null pointer to the trait’s vtable if implemented
• Null pointer if the trait is not implemented

Safety

• ptr must be a valid pointer to mx_erased
• The returned pointer (if non-null) must be cast to the correct vtable type

data_offset: usize

Byte offset from the start of the wrapper struct to the data field.

The generated wrapper struct is #[repr(C)] struct { erased: mx_erased, data: T }. When T has stricter alignment than mx_erased, padding exists between erased and data. This field records the correct offset so that TraitView::try_from_sexp can compute the data pointer without assuming offset == size_of::<mx_erased>().