The parent argument in the uvm_component constructor - system-verilog

Question

I expected my_child to inherit the say_hello function from my_parent in the following code, but it did not.
Can someone explain to me what exactly parent argument does?
class my_parent extends uvm_component;
`uvm_component_utils(my_parent);
function new(string name = "my_parent", uvm_component parent);
super.new(name, parent);
endfunction: new
function void say_hello;
$display("Hello, UVM!");
endfunction: say_hello
endclass: my_parent
/*========================================*/
class my_child extends uvm_component;
`uvm_component_utils(my_child);
function new(string name = "my_child", uvm_component parent);
super.new(name, parent);
endfunction: new
endclass: my_child
/*========================================*/
module top;
my_parent p;
my_child c;
initial begin
p = my_parent::type_id::create("p", null);
c = my_child::type_id::create("c", p);
c.say_hello;
end
endmodule: top

Answer 1

You are mixing up the terms parent and child with the principals of class inheritance. They are distinct objects. A parent creates a child, and the uvm_component represents a hierarchal family tree in a database. Each component you create has a handle to its parent, and the parent has a list of handles that are its children.
class my_child extends uvm_component;
`uvm_component_utils(my_child);
function new(string name = "my_child", uvm_component parent);
super.new(name, parent);
endfunction: new
endclass: my_child
class my_parent extends uvm_component;
`uvm_component_utils(my_parent);
my_child child;
function new(string name = "my_parent", uvm_component parent);
super.new(name, parent);
endfunction: new
function void build_phase(uvm_phase phase;
child = my_child::type_id::create("child", this); // 'this' is the parent object
endfunction : build_phase
endclass: my_parent
module top;
my_parent p;
initial begin
p = my_parent::type_id::create("p", null);
end
endmodule: top
When using inheritance, we have entended or derived classes, they are not the children of the base class.
import uvm_pkg::*;
`include "uvm_macros.svh"
class my_child extends uvm_component;
`uvm_component_utils(my_child);
function new(string name = "my_child", uvm_component parent);
super.new(name, parent);
endfunction: new
task run_phase(uvm_phase phase);
$display("Hello, UVM!, from base");
endtask: run_phase
endclass: my_child
class my_parent extends uvm_component;
`uvm_component_utils(my_parent);
my_child child;
function new(string name = "my_parent", uvm_component parent);
super.new(name, parent);
endfunction: new
function void build_phase(uvm_phase phase);
child = my_child::type_id::create("cchild", this); // 'this' is the parent object
endfunction : build_phase
endclass: my_parent
class my_child_extended extends my_child;
`uvm_component_utils(my_child_extended);
function new(string name = "my_child", uvm_component parent);
super.new(name, parent);
endfunction: new
endclass: my_child_extended
module top;
my_parent p;
initial begin
my_child::type_id::set_type_override(my_child_extended::get_type());
run_test("my_parent");
end
endmodule: top

Answer 2

You should extend the my_child class from the my_parent class instead of uvm_component when you want to inherit from the my_parent class. Change:
class my_child extends uvm_component;
to:
class my_child extends my_parent;
When you make this change and run the simulation, it prints the following, as expected:
Hello, UVM!
The parent variable is explained in the UVM Class Reference document under uvm_component.