Re: Need a way to convert a constant to a variable

[Victor Porton <porton@narod.ru>]

Dmitry A. Kazakov wrote:

> On 2017-08-05 15:41, Victor Porton wrote:
>> 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;
> 
> AFAIK, Ada.Containers.Indefinite_Holders was designed in a way to
> prevent specifically what you want.
> 
>> Can any problem with different representation of constant and variables
>> appear?
> 
> 1. Don't use Ada.Containers.Indefinite_Holders?
> 
> 2. Don't use char_array. It has restricted use when interfacing C. You
> probably need chars_ptr put into a controlled type if you want safe C
> strings. This is basically same as Holder internally is, except that you
> are in full control.

I do use char_array for interfacing with C.

It is a pity that I need to reimplement Indefinite_Holders for this task.

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