RE: [sv-ec] "const ref" for parameters and consts

It may not be clear from the LRM text, but the intent is that a const variable can be passed to a const ref argument. The only restriction on a const variable is that it cannot be written. Since you cannot write to a const ref argument, passing a const variable to a const ref argument cannot result in its being written, so it would be legal. One reason this might not have been explicitly specified is that the const qualifier was borrowed from C, and the authors of the LRM text may have automatically assumed the behavior from C without specifying it in the LRM.

Parameters are another matter, as they are not just variables you aren't allowed to write to. An implementation may not keep a copy of the value available in the format that variables are stored in, suitable for passing to a ref argument. Doing so would only be a minor burden on the implementation. But it is also trivial for the user to set a const variable to the value of the parameter and pass that. I don't think we want to start down that slippery slope that leads to arbitrary expressions being passed to ref arguments.

From: owner-sv-ec@eda.org [mailto:owner-sv-ec@eda.org] On Behalf Of Daniel Mlynek
Sent: Wednesday, August 24, 2011 8:31 AM
To: sv-ec@eda.org
Subject: [sv-ec] "const ref" for parameters and consts

LRM about refs says:
Only the following shall be legal to pass by reference:
- a variable,
- a class property,
- a member of an unpacked structure, or
- an element of an unpacked array.

So only variables are allowed for ref ports (no word about const, parameter)
But then const ref was added to the standart.
IMHO there is no sensible argument to forbid parameters to be passed by "const ref" like in below case.

module top;
 parameter byte a = 1;
 task automatic t (const ref byte a);
  $display(a);
 endtask
 initial
  t(a);
endmodule

Maybe LRM should allow parameters and const explicitly for const ref only.

Should I register a mantis - there is no similar yet?

DANiel

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