GNAT Unchecked_Union and multiple components

GNAT Unchecked_Union and multiple components

[Brian]

Hi,

I've started to use GNAT 3.15p and noticed it handles the pragma
Unchecked_Union differently from other compilers.  According to the
GNAT Reference Manual "Each variant has a component list with a single
component.", if you violate this rule the compiler will generate the
error "Unchecked_Union variant can have only one component".  Other
compilers allow multiple components per variant.  Is there a GNAT
compiler switch I can use to loosen this restriction?  My current
workaround is to create a new record type containing the multiple
components and use it as the type for one of the variants (see code
snippet below).  Thanks.

Regards,
Brian


<snip>

type Msg_Type is (Msg_A, Msg_B);

-- This is what I want as my variant record type definition.

type Data_Type1(Msg : Msg_Type := Msg_A) is
   record
      case Msg is
         when Msg_A =>
            A1 : Integer;
            A2 : Integer;
         when Msg_B =>
            B : Float;
      end case;
   end record;

-- This is my current workaround.

type Msg_A_Variant is
   record
      A1 : Integer;
      A2 : Integer;
   end record;

type Data_Type1(Msg : Msg_Type := Msg_A) is
   record
      case Msg is
         when Msg_A =>
            A : Msg_A_Variant;
         when Msg_B =>
            B : Float;
      end case;
   end record;

-- end workaround

pragma Unchecked_Union(Data_Type1);

<snip>

Re: GNAT Unchecked_Union and multiple components

[Randy Brukardt]

"Brian" <brian.ngc@gmail.com> wrote in message
news:1113836280.439166.180090@f14g2000cwb.googlegroups.com...
> Hi,
>
> I've started to use GNAT 3.15p and noticed it handles the pragma
> Unchecked_Union differently from other compilers.  According to the
> GNAT Reference Manual "Each variant has a component list with a single
> component.", if you violate this rule the compiler will generate the
> error "Unchecked_Union variant can have only one component".  Other
> compilers allow multiple components per variant.

As does the upcoming Ada 2006.

> Is there a GNAT
> compiler switch I can use to loosen this restriction?

Not with GNAT 3.15p, but I believe the current versions of GNAT (which
others here can explain how to get better than I can) in fact support
Unchecked_Union as in the proposed Amendment to Ada (commonly known as Ada
2006 or Ada 2005).

             Randy Brukardt