!standard 4.6(24.12/2) 17-09-05 AC95-00292/00 !class Amendment 17-09-05 !status received no action 17-09-05 !status received 17-08-05 !subject Need a way to convert a constant to a variable !summary !appendix !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? *************************************************************** From: Martin Dowie Sent: Monday, August 7, 2017 7:29 AM 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 begin return To_Chars_Ptr (Constant_Reference(Object).Element); end To_C_String; begin null; end; *************************************************************** From: Victor Porton Sent: Monday, August 7, 2017 3:19 PM You are right, your code does what I need. I've rewritten my actual free software project at which I encountered the trouble with your advice. This time I was a fool. I tried a few hours and found no solution of my particular problem. However, my idea that sometimes one may need to change a constant view into a variable view, it may be nevertheless worth. It is dangerous, but I proposed the attribute started from "Unchecked_" for it, so the user should know what he does. ***************************************************************