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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Is this actually possible?
Date: Wed, 11 Dec 2019 18:38:34 +0100
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Date: 2019-12-11T18:38:34+01:00
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On 2019-12-11 17:43, Lucretia wrote:

> I was thinking about extensible records recently (not tagged types), and thought to try to export a tagged type to C, not C++. It compiles, but the results aren't quite right, so wondering if it's possible or not.
> 
> The idea is to export an array of tagged types as a C array to either a function or a variable / struct element. i.e.

Ada objects of tagged types have varying size and keep tag inside. Both 
make them utterly incompatible with C.

>     type Storage_Array is array (Positive range <>) of Storage_Element with
>       Convention => C;

This type cannot be used with C because an object of will have bounds 
inside.

If you need an array compatible with C, it must a "flat" array:

    type C_Storage_Array is array (size_t) of Storage_Element;

Naturally you will never declare such objects in your program. You will 
get them from C or allocate raw memory and then having the address of 
the first element do:

    A : C_Storage_Array;
    pragma Import (Ada, A);
    for A'Address use The_Address_Of_A;

It is your responsibility not to index it outside the bounds. Just like 
in C.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de