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From: Ian Clifton <ian@melvin.dp.ox.ac.uk>
Subject: Re: CONTRAINT ERROR (was Re: parsing a string)
Date: 2000/01/26
Message-ID: <4qaeltvtop.fsf@melvin.dp.ox.ac.uk>#1/1
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Newsgroups: comp.lang.ada
Date: 2000-01-26T13:39:05+00:00
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pumilia@est.it writes:

>  output_file_string : unbounded_string ;
>  output_file : string (1..25) ;
>  pos1 : natural;
> 
> Begin
> 
>     get_line(output_file_string);
> 
>    put(" output:  "); put(output_file_string); put("<--"); new_line;
>    output_file :=  to_string(output_file_string);
>    put(" Output:  "); put(output_file); put("<--"); new_line;
>    put(" index:  "); pos1 := Index(output_file,"xxx");
>    put(pos1);
>    new_line;
> 
> end str2_test;
> -- str2_test --------------------------------------
> 
> 
> While executing i get the error:
> 
> >   raised constraint_error : str2_test.adb:24
> 
> This is line 24:
> 
>   output_file :=  to_string(output_file_string);
> 
> 
> Any help wil be appreciiated
> thank you
> Pol

I think the problem is that output_file is a fixed-length(=25) string,
while Ada.Strings.Unbounded.To_String returns a fixed string of
whatever length output_file_string happens to be. This will fail
unless that length happens to be 25. In places where you'd like to
assign to a fixed-length string, it almost always makes more sense to
use Ada.Strings.Fixed.Move instead. Ada.Strings.Fixed.Move pads or
truncates appropriately to match the length of the target string.

-- 
Ian Clifton                   Phone: +44 1865 275631
Dyson Perrins Laboratory      Fax:   +44 1865 275674
Oxford University  OX1 3QY UK ian.clifton@linacre.ox.ac.uk