$cast() works but variable can't use

SystemVerilog
,
1

Trying to understand $cast and Factory. I made a simple example from reference code.

Reference code

class base;
  int unsigned b;
endclass : base

class derived extends base;
  int unsigned d;
endclass : derived

module top();
  initial begin
  base B;
  derived D;
    
    D = new();

    D.b = 'hB;
    D.d = 'hD;
    B = D; 
    
    if ($cast(D, B)) begin
      $display ("D.b:%0h", D.b); 
      $display ("D.d:%0h", D.d);
    end
  end
endmodule : top

From reference code, I get it the $cast() functionality which I can use a variable “b” from D.

So I made two example for understand casting under overriding.

  1. $cast() with override.

  import uvm_pkg::*; 
`include "uvm_macros.svh" 
  
  class agent_a extends uvm_agent; 
    `uvm_component_utils(agent_a) 
    int a=1; 
  
    function new(string name = "agent_a", uvm_component parent = null); 
      super.new(name, parent); 
    endfunction : new 
  endclass : agent_a 
  
  class agent_b extends agent_a; 
    `uvm_component_utils(agent_b) 
    int b=2; 
  
    function new(string name = "agent_b", uvm_component parent = null); 
      super.new(name, parent); 
    endfunction : new 
  endclass : agent_b 
  
  class agent_c extends agent_b; 
    `uvm_component_utils(agent_c) 
    int c=3; 
  
    function new(string name = "agent_c", uvm_component parent = null); 
      super.new(name, parent); 
    endfunction : new 
  endclass : agent_c 
 
module test_module (); 
  initial begin 
 
  agent_a agent_a_h; 
  agent_b agent_b_h; 
  agent_c agent_c_h; 
 
  int cast1; 
  
    agent_a::type_id::set_type_override(agent_b::get_type()); 
    factory.print(); 
    agent_a_h = agent_a::type_id::create("agent_a_h", null); 
    agent_a_h = agent_c_h; 
 
    cast1 = $cast(agent_c_h, agent_a_h);  
    $display("casts =%01d,   agent_c_h.a=%01d, agent_c_h.b=%01d ", cast1, agent_c_h.a, agent_c_h.b );

In “2. $cast() with override”, cast success. but I have faced that *E,TRNULLID: NULL pointer dereference. I expected that I can downcast by $cast(agent_c_h, agent_a_h); but why I got faced NULL pointer dereference error about agent_c_h?

2

In reply to UVM_LOVE:

The issue is with the way you create()/assign your class handles:


    agent_a::type_id::set_type_override(agent_b::get_type()); // Sets the override for agent_a to agent_b
    factory.print(); 
    agent_a_h = agent_a::type_id::create("agent_a_h", null);  // Creates agent_a_h with type of agent_b
    agent_a_h = agent_c_h; // Assigns agent_c_h to agent_a_h, but agent_c_h is 'null'
 
    cast1 = $cast(agent_c_h, agent_a_h);  // Casts agent_a_h (null) to agent_c_h, which results in null.