instance_variable_defined?

/*
 *  call-seq:
 *     obj.instance_variable_defined?(symbol)    => true or false
 *
 *  Returns <code>true</code> if the given instance variable is
 *  defined in <i>obj</i>.
 *
 *     class Fred
 *       def initialize(p1, p2)
 *         @a, @b = p1, p2
 *       end
 *     end
 *     fred = Fred.new('cat', 99)
 *     fred.instance_variable_defined?(:@a)    #=> true
 *     fred.instance_variable_defined?("@b")   #=> true
 *     fred.instance_variable_defined?("@c")   #=> false
 */

static VALUE
rb_obj_ivar_defined(obj, iv)
    VALUE obj, iv;
{
    ID id = rb_to_id(iv);

    if (!rb_is_instance_id(id)) {
        rb_name_error(id, "`%s' is not allowed as an instance variable name", rb_id2name(id));
    }
    return rb_ivar_defined(obj, id);
}