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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Variant record memory storage question
Date: Wed, 1 Aug 2018 20:58:46 +0200
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On 01.08.18 19:38, NiGHTS wrote:
> Will the record Variant_Record( Is_Small ) be stored as 1 byte? Or does it act almost like a C union allocating the memory for Variant_Record( Is_Big ) even though it will never morph at run-time?

(You could also try pragma Unchecked_Union.)

GCC's -gnatRn has hints that will help answer the question.
Using defaults for representation,

package Rep is

     type Byte is mod 512;

     type Some_Large_Array is array (1 .. 1_000) of Byte;

     type Some_Large_Record is
        record
            Data : Some_Large_Array;
        end record;


     type Which_One is (Is_Small, Is_Big);

     type Variant_Record (Option : Which_One) is
        record
            case Option is
               when Is_Small =>
                   Var_1 : Byte; -- 1 byte
               when Is_Big =>
                   Var_2 : Some_Large_Record; -- 1000 bytes
            end case;
        end record;

     S : Variant_Record (Is_Small);

     B : Variant_Record (Is_Big);

end Rep;


$ gcc -c -x ada -gnatR2 rep.ads

Representation information for unit Rep (spec)
----------------------------------------------

for Byte'Object_Size use 16;
for Byte'Value_Size use 9;
for Byte'Alignment use 2;

for Some_Large_Array'Size use 16000;
for Some_Large_Array'Alignment use 2;
for Some_Large_Array'Component_Size use 16;

for Some_Large_Record'Size use 16000;
for Some_Large_Record'Alignment use 2;
for Some_Large_Record use record
    Data at 0 range  0 .. 15999;
end record;

for Which_One'Object_Size use 8;
for Which_One'Value_Size use 1;
for Which_One'Alignment use 1;

for Variant_Record'Object_Size use 16016;
for Variant_Record'Value_Size use ??;
for Variant_Record'Alignment use 2;
for Variant_Record use record
    Option at 0 range  0 ..  7;
    Var_1  at 2 range  0 .. 15;
    Var_2  at 2 range  0 .. 15999;
end record;

for S'Size use 32;
for S'Alignment use 4;

for B'Size use 16016;
for B'Alignment use 2;

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