return unconstrained array in vhdl

Question

what's the best way to return an unconstrained vector in vhdl?

function func(selector : natural) return std_logic_vector is
begin
    case selector is
        when 3 => return std_logic_vector("11");
        when 4 => return std_logic_vector("101");
        when others => return std_logic_vector("0");
    end case;
end function;

In this case i get string literal cannot be a type conversion operand, so it doesn't work. The signal selector is generic, so it don't have to be synthesizeable.

You have syntax errors after numeric literals 3 and 4. Do you mean to use a qualified expression which explicitly specifies a type? Qualified expressions: std_logic_vector'("11"), std_logic_vector'("101"); and std_logic_vector'("0"). The error message you haven't shown would tell you that you can't type convert a string to std_logic_vector, their element types aren't the same (character and std_ulogic) . Those ':'s should be "=>". — user1155120
– user1155120, Commented Oct 10, 2017 at 13:14
The : instead of <= was a mistake in here, i typed this part new for stackexchange, without unnecessary code. In the original code there're also "<=". However, it works with the ' you mentioned before the brackets.Thanks a lot. Can you give me a hint about how's this ' function called in this case? Copy this in an answer, i'll mark as accepted! — Franz Forstmayr
– Franz Forstmayr, Commented Oct 10, 2017 at 13:40
You still don't have a viable code snippet in your question. You should be using right arrow => for associating a choice with the following return statement. when 3 => return std_logic_vector'("11");, etc. Please supply a Minimal, Complete and Verifiable example. Include a complete error message. Your question is not useful as it stands. Note that the return value type is std_logic_vector and can provide the type of the string literals without the qualified expressions as Matthew Taylor commented to his answer. — user1155120
– user1155120, Commented Oct 10, 2017 at 19:49

Matthew · Accepted Answer · 2017-10-10 12:31:16Z

1

You can't do this. The return value from your function will need to be associated with something when it is called and that something will have to be of a fixed width. In other words, you'll have to say thing like:

s <= func(n);

and s will have a fixed width, so all the return values from your function will have to have the same width.

answered Oct 10, 2017 at 12:31

Matthew

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4 Comments

Franz Forstmayr Over a year ago

My function return value is used in a generic map with an unconstrained array type. Like generic map(func(n)). The width of the generic is evaluated in lower hierarchy levels.

Matthew Over a year ago

@FranzForstmayr In which case, I don't understand what you're asking. You can just say this when 3 => return "11";.

Franz Forstmayr Over a year ago

Ok, i didn't knew that this is possible. I saw, i just thought much too complicated!

user1155120 Over a year ago

"The return value from your function will need to be associated with something when it is called and that something will have to be of a fixed width." No, the return value must be evaluated, as it would be in the OP's intended use as an initial value for a (generic) constant where the subtype indication does not supply a constraint. His MCVe could also provide usage for func.

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