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Newsgroups: comp.lang.ada
Date: Sat, 26 May 2018 14:40:41 -0700 (PDT)
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Subject: Re: why "unconstrained subtype in component declaration" while said
 component given default value ?
From: Jere <jhb.chat@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:52705
Date: 2018-05-26T14:40:41-07:00
List-Id: <comp.lang.ada>

On Saturday, May 26, 2018 at 5:03:23 PM UTC-4, Mehdi Saada wrote:
> Hello.
> Why is this case not allowed ? Is there really an implementation issue here ? I doubt so. Why should it be any different from the non-issue of initializing variables of unconstrained subtypes vs declaring uninitialized variables of constrained subtypes ?
>  type A (N: Positive) is record
>       II : Integer;
>       Chaine : String := Give_String(N);
>    end record;
> 

Consider for a second that you are the compiler and you need to 
allocate a variable:

My_Variable : A(10);

What size (in bytes) do you need to allocate for the variable?  Remember
that the function Give_String can be exceedingly complex or even a 
function from a shared library.  How do you at compile time determine
the size of that record type?

However you can do:

    type A(N : Positive) is record
        II      : Integer;
        Chaine  : String(1..N) := Get_String(N);
    end record;

Because now the compiler will know at compile time how long the string
is and thus how big to make the record.