Override instance

In reply to Huzefa Halolwala:
I have design one simple testbench in which there are two driver classes

  1. driver (drv)
  2. driver2 (drv2)
    and I want to override the instance with driver2 (drv2), But it is showing me the result as drv not the replaced one drv2. This is what i have written in my testcase

 
driver::type_id::set_inst_override(driver2::get_type(),"top.t_env.ag2.drv2");