Ruby::Box.new

static VALUE
box_initialize(VALUE box_value)
{
    rb_box_t *box;
    rb_classext_t *object_classext;
    VALUE entry;
    ID id_box_entry;
    CONST_ID(id_box_entry, "__box_entry__");

    if (!rb_box_available()) {
        rb_raise(rb_eRuntimeError, "Ruby Box is disabled. Set RUBY_BOX=1 environment variable to use Ruby::Box.");
    }

    entry = rb_class_new_instance_pass_kw(0, NULL, rb_cBoxEntry);
    box = get_box_struct_internal(entry);

    box->box_object = box_value;
    box->box_id = box_generate_id();
    rb_define_singleton_method(box->load_path, "resolve_feature_path", rb_resolve_feature_path, 1);

    // Set the Ruby::Box object unique/consistent from any boxes to have just single
    // constant table from any view of every (including main) box.
    // If a code in the box adds a constant, the constant will be visible even from root/main.
    RCLASS_SET_PRIME_CLASSEXT_WRITABLE(box_value, true);

    // Get a clean constant table of Object even by writable one
    // because ns was just created, so it has not touched any constants yet.
    object_classext = RCLASS_EXT_WRITABLE_IN_BOX(rb_cObject, box);
    RCLASS_SET_CONST_TBL(box_value, RCLASSEXT_CONST_TBL(object_classext), true);

    rb_ivar_set(box_value, id_box_entry, entry);

    return box_value;
}