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From: Victor Porton <porton@narod.ru>
Newsgroups: comp.lang.ada
Subject: Need a way to convert a constant to a variable
Date: Sat, 05 Aug 2017 16:41:23 +0300
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Date: 2017-08-05T16:41:23+03:00
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I've sent the following email to ada-comment mailing list. I duplicate it 
here.

!topic Need a way to convert a constant to a variable
!reference Ada 2012 RM
!from Victor Porton 17-08-05
!keywords constant, variable, view, conversion
!discussion

Sometimes one needs to convert a constant view into variable view (I am
fully conscious that after this the programmer should take care not to
change the object of the view).

In the following (not compilable with GNAT 7.1.0) code I present my
best attempt to solve the following problem:

Write a function with an "in" indefinite holder with a string, return
chars_ptr corresponding to the string.

It looks like there is no solution of this in Ada 2012 :-(

with Interfaces.C; use Interfaces.C;
with Interfaces.C.Strings; use Interfaces.C.Strings;
with Ada.Containers.Indefinite_Holders;

procedure Conv is

   package Char_Array_Holders is
      new Ada.Containers.Indefinite_Holders(char_array);

   type C_String_Holder is new Char_Array_Holders.Holder
      with null record;
   
   function To_C_String (Object: C_String_Holder) return chars_ptr is
      Value: char_array renames Constant_Reference(Object).Element.all;
      Value2: aliased Char_Array(Value'Range) with Import;
      for Value2'Address use Value'Address;
   begin
      return To_Chars_Ptr(Char_Array_Access'(Value2'Access));
   end;
begin
   null;
end;

$ gnatgcc -c conv.adb -c conv.adb 
conv.adb:13:07: warning: aliased object has explicit bounds
conv.adb:13:07: warning: declare without bounds (and with explicit
initialization)
conv.adb:13:07: warning: for use with unconstrained access
conv.adb:16:46: object subtype must statically match designated subtype

My current workaround is to define my own "indefinite holder" type to
use it in my software instead of Ada.Containers.Indefinite_Holders.

I propose the following language change:

Please add 'Unchecked_Variable attribute.

There are two possible meanings (of which we should choose one) of the
attribute:

1. C'Unchecked_Variable returns a variable view of a constant C.

2. A'Unchecked_Variable returns the corresponding access to a variable
for an access A to constant.

Both variants seem to solve the trouble.

Can any problem with different representation of constant and variables
appear?

-- 
Victor Porton - http://portonvictor.org