Ada - how to initialize a limited tagged type that contains File_Type members? - oop

Question

In the following code Mix_Card_Reader inherits from Mix_IO_Device, the latter being an abstract tagged record.
Previously it contained one Positive and two Stream_Access members. I'd like to alter the code so that it uses File_Type members instead.
The reason for that is that I want each instance of this type to be able to open and close its files as and when required, or not at all if need be.
The problem is that I cannot initialise this inheriting type because File_Type is a limited type. How can I write my Create_Mix_Card_Reader function to allow this?
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type Mix_IO_Device is abstract tagged limited
record
Block_Size : Positive;
Input_File : File_Type;
Output_File : File_Type;
end record;
type Mix_Card_Reader is new Mix_IO_Device with null record;
.adb...
function Create_Mix_Card_Reader return Mix_IO_Device_Access is
Ret : Mix_IO_Device_Access := new Mix_Card_Reader'(16, null, null);
begin
return Ret;
end Create_Mix_Card_Reader;
GNAT is complaining that I cannot pass null, null into the pair of File_Type members because they are not compatible of course, the nulls are a left-over from when this used to have Stream_Access members. It seems that I have to pass something in here but I don't want to have to prematurely open the files simply to placate the compiler.
What to do?
Edit:
I have a couple of obvious options:
use access File_Type instead (but I still have to maintain the opening/closing of the files elsewhere).
store all the File_Type objects in an array separately and just refer to them using Streams as before but this seems messy.

Answer 1

This should do the trick:
function Create_Mix_Card_Reader return Mix_IO_Device_Access is
Ret : Mix_IO_Device_Access := new Mix_Card_Reader'(
16, Input_Type => <>, Ouptut_Type => <>);
begin
return Ret;
end Create_Mix_Card_Reader;
The box notation is a placeholder for the default value. You need at least Ada 2005 to use it in aggregates and must not use positional notation, details are explained in the Ada 2005 Rationale. You can shorten the two assignments to others => <> if you want.

Answer 2

You don’t really need to initialize the File_Type variables, since they start off initialized (but not opened).
I got the impression that you didn’t start off using an access type? Try this (not an answer to the question as posed, but may still be useful):
with Ada.Text_IO; use Ada.Text_IO;
package Wossname is
type Mix_IO_Device is abstract tagged limited
record
Block_Size : Positive;
Input_File : File_Type;
Output_File : File_Type;
end record;
type Mix_Card_Reader is new Mix_IO_Device with null record;
function Create_Mix_Card_Reader return Mix_IO_Device'Class;
end Wossname;
I’m not 100% sure of the exact legality here, but I think this is "initializing in place":
package body Wossname is
function Create_Mix_Card_Reader return Mix_IO_Device'Class is
begin
return Ret : Mix_Card_Reader do
Ret.Block_Size := 16;
end return;
end Create_Mix_Card_Reader;
end Wossname;
and as you can see it compiles (and runs!) OK.
procedure Wossname.Test is
Reader : Mix_IO_Device'Class := Create_Mix_Card_Reader;
begin
begin
Create (Reader.Output_File, Name => "wossname.out", Mode => Out_File);
exception
when Use_Error =>
Open (Reader.Output_File, Name => "wossname.out", Mode => Out_File);
end;
Put (Reader.Output_File, "hi!");
Close (Reader.Output_File);
end Wossname.Test;