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>().

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 40 bytes