integer_vector as a NULL array

Question

How do I pass an empty integer_vector with length equals zero? (= NULL array)

Background:

Inside a Testbench I´d like to use a procedure for different types of transactions. Something like this:

entity foo is
end entity foo;

architecture bar of foo is
  procedure Transaction (
        constant RX_BYTES     : integer_vector
        ) is
    begin
        report "RX_BYTES'length = " & to_string(RX_BYTES'length);
    end procedure;
begin

  Transaction(RX_BYTES => ""); -- it´s about this line

end architecture bar;

I have something like this in mind, but that doesn´t work:

Transaction(RX_BYTES => "");

ModelSim error:

** Error: C:/Users/sbuhrow/Desktop/NULL_Vector.vhd(13): (vcom-1600) No feasible entries for subprogram "Transaction".

Would you mind to provide a minimal reproducible example, please? — the busybee
– the busybee, Commented Jan 5, 2024 at 10:56
@am9417 Did you visit the linked page? We need to copy and paste the source to reproduce. Currently this is not possible. — the busybee
– the busybee, Commented Jan 5, 2024 at 16:00
@thebusybee As the question is about something the OP don't know how to do they cannot really provide an interesting MCVE. MCVE are nice when people want help with unexpected results, but here there is no result at all because they don't know how to code what they need. — Renaud Pacalet
– Renaud Pacalet, Commented Jan 5, 2024 at 16:57
@RenaudPacalet As your answer proves, there is a minimal reproducible example. With the form the OP chose, it will produce an error. But that is fine. It gives us a starting point. — the busybee
– the busybee, Commented Jan 5, 2024 at 19:01

Renaud Pacalet · Accepted Answer · 2024-01-05 14:31:00Z

5

Simply use an empty range:

Transaction(RX_BYTES => (1 to 0 => 0), ...);

Demo:

$ cat foo.vhd
entity foo is
end entity foo;

architecture bar of foo is
  procedure p(constant c: integer_vector) is
  begin
    report "c'length = " & to_string(c'length);
  end procedure p;
begin
  p(c => (0 downto 1 => 0));
end architecture bar;

$ ghdl -a --std=08 foo.vhd
$ ghdl -r --std=08 foo
foo.vhd:7:5:@0ms:(report note): c'length = 0

Note: if you will use such an empty integer vector frequently you could also declare a constant:

constant empty_iv: integer_vector := (1 to 0 => 0);

And use it everywhere you need it.

edited Jan 5, 2024 at 14:31

answered Jan 5, 2024 at 14:20

Renaud Pacalet

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1 Comment

El-Coder-SB Over a year ago

That works :-) I needed some time to understand that the key is the intentional "wrong" use of indices: e.g. (0 downto 1) has the lower number on the left side which results in the wanted null range vector.

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