VHDL: Using array of std_logic vs. using subtype of std_logic_vector

Question

I have multiple inputs that belong together(get sampled at the same clock etc.) but are not logically a vector(i.e. not a parallel bus) in a piece of existing code I need to modify.
Previously, they were defined as

type my_type is array (my_width - 1 downto 0) of std_logic;
signal my_signal : my_type;

Until now, for this purpose, I always used this:

subtype my_subtype is std_logic_vector(my_width - 1 downto 0);
signal my_signal : my_subtype;

For most intentions and purposes, an array and a vector can be handled pretty much the same, so my question is:
Is there any advantage to either way of doing things? Is there a preferred/standard way?

Answer 1

There is a difference, which has to do with strong typing.

With your first piece of code, you create a new type. It is independent and not compatible with other types. Just consider the next piece of code:

entity e is end entity;
library ieee;
architecture a of e is
    use ieee.std_logic_1164.all;
    type slv1_t is array (15 downto 0) of std_logic;
    signal slv1 : slv1_t;
    type slv2_t is array (7 downto 0) of std_logic;
    signal slv2 : slv2_t;
begin
    slv2 <= slv1(7 downto 0);
end architecture;

When compiling in modelsim, this code will give errors:

Error: C:/HDL/TypeVsSubtype/Type.vhd(10): Cannot resolve slice name as type slv2_t.

For the second piece of code, the underlying type is still a std_logic_vector. Thus the subtypes are compatible. Consider the next piece of code:

entity e is end entity;
library ieee;
architecture a of e is
    use ieee.std_logic_1164.all;
    subtype slv1_t is std_logic_vector(15 downto 0);
    signal slv1 : slv1_t;
    subtype slv2_t is std_logic_vector(7 downto 0);
    signal slv2 : slv2_t;
begin
    slv2 <= slv1(7 downto 0);
end architecture;

This does compile (i.e. no errors).