Issue with usage of const input in methods

UVM
,
1

For the below code ::


class Base ;

virtual function void DISP ( const input int A  ) ; 

  $display("In Base");

endfunction  

endclass

class Ext extends Base ;

virtual function void DISP ( const input int A ) ; 
                                                      
  $display("In Ext");

endfunction  

endclass

Ext e ; 
Base b ;

int a = 10 ;

initial begin

 e = new();
 b = e;
 b.DISP(a);

end

I see following error :: The keyword ‘const’ must be followed by ‘ref’ in formal argument lists.

IN SV LRM Section 13.3 I do see syntax :: tf_port_direction ::= port_direction | const ref . So this tells that legal port direction could be input / output / input / ref / constant ref .

Now the reason I ask the syntax is during user_subscriber classes when I use ::


// In class user_subs extends uvm_subscriber # ( trans ) ;
function void write ( const input trans t ) ;
 ....

I get same error . I want const input so that transactions are not modified by any of the subscriber class

So why is it illegal to use " const input " ?

2

In reply to MICRO_91:

A const ref argument is a guarantee to the caller that you will not modify the actual argument passed to the function. You cannot modify the actual argument of an input to a function because it’s copied upon calling the function and not copied when it returns.

This would not help you if even if it were legal because the argument is only a handle to the transaction. With const ref, you cannot modify the handle, but you can still write to object members that handle references.

In UVM, we use the MO-COW principle - Manual Object Clone On Write.

3

In reply to dave_59:

Hi Dave ,

SV LRM 13.5.2 says :: " When the formal argument is declared as const ref , the subroutine cannot alter the variable , and an attempt to do so shall generate a compile error " .

I am confused by what it means by “cannot alter the variable” when we talk about objects .
I understand that we don’t discuss simulator issues here but I see different results on simulators .

Those which flash error do no allow to change the contents of the object ( All flash an error on usage of const ref and using new() on the object passed as argument )


function void DISP ( cons ref trans t ) ;
    // t = new() ; // Error 
     t.a = 100 ;  // Modifying contents of object has conflicting Outputs on Simulators 
.....
endfunction
4

In reply to MICRO_91:

In your code, t is a class variable that holds a handle. That handle is a reference to an object. The object is not a variable, it’s a container to a group of member variables and methods.

On simulators that claim it should be an error when writing to a member variable, run the example below. You will see that it’s a pointless error since it’s so easy to get around it.

class trans;
  int a;
endclass
module top;
  function automatic void f(const ref trans tx);
    trans rx = tx;
    // tx.a = 10; // is legal according to the LRM
    rx.a=10;
  endfunction
  trans h;
  initial begin
    h = new;
    f(h);
    $display(h.a);
  end
endmodule