Ada constant array of string literals - arrays

Question

I have a large array in C that I wish to move into my ada project. That array is used to store filenames for assets which will later be loaded. It looks something like:
const char *filenames[NUMBER_OF_FILES] = {
/* file[0] */ "res/0.file",
/* ... */
/* file[n] */ "res/some_more/complicated/file.name"
};
I want to move this into an ada package body, but can't find a decent way to do it. Obviously my first attempt was:
filenames : constant array (File_Index) of String := (
index_0 => "res/0.file",
-- ...
index_n => "res/some_more/complicated/file.name"
);
But of course String is an unconstrained type, so Ada won't allow that. I switched it to use Unbounded_Strings, which worked, but was very ugly (having to wrap each string with To_Unbounded_String.
Is there any way to make an array of unconstrained types whose size will be known at compile time like this, or do I have to use unbounded strings?

Answer 1

It’s a bit low-level and repetitive, but perhaps you can create a little program (maybe even in Ada!) to generate something like
with Ada.Text_IO; use Ada.Text_IO;
procedure Lambdabeta is
F1 : aliased constant String := "res/0.file";
F2 : aliased constant String := "res/some_more/complicated/file.name";
type Strings is array (Positive range <>) of access constant String;
Filenames : constant Strings := (F1'Access,
F2'Access);
begin
for J in Filenames'Range loop
Put_Line (Filenames (J).all);
end loop;
end Lambdabeta;
See also this answer on minimising the pain of using To_Unbounded_String.

Answer 2

Arrays can't contain objects of indefinite types.
Basically you have two options:
Use another container than an array.
Encapsulate the strings in a definite type.
So you could use Ada.Containers.Indefinite_Vectors instead of an array:
with Ada.Containers.Indefinite_Vectors;
package Settings is
------------------------------------------------------------------
-- You may want to put this block in a separate package:
package String_Vectors is
new Ada.Containers.Indefinite_Vectors (Index_Type => Positive,
Element_Type => String);
function "+" (Left, Right : String) return String_Vectors.Vector
renames String_Vectors."&";
function "+" (Left : String_Vectors.Vector;
Right : String) return String_Vectors.Vector
renames String_Vectors."&";
------------------------------------------------------------------
File_Names : constant String_Vectors.Vector :=
"/some/file/name" +
"/var/spool/mail/mine" +
"/etc/passwd";
end Settings;
So you could use Ada.Strings.Unbounded.Unbounded_String instead of String:
with Ada.Strings.Unbounded;
package Settings_V2 is
function "+" (Item : in String) return Ada.Strings.Unbounded.Unbounded_String
renames Ada.Strings.Unbounded.To_Unbounded_String;
type String_Array is array (Positive range <>)
of Ada.Strings.Unbounded.Unbounded_String;
File_Names : constant String_Array :=
(+"/some/file/name",
+"/var/spool/mail/mine",
+"/etc/passwd");
end Settings_V2;

Answer 3

What has not yet been mentioned is that you can just use a function:
subtype File_Index is Integer range 1 .. 3;
function Filename (Index : File_Index) return String is
begin
case Index is
when 1 => return "res/0.file";
when 2 => return "res/1.file";
when 3 => return "res/some_more/complicated/file.name";
end case;
end Filename;
Using Filename (1) in your code is identical to accessing an array element.