Generics & Elaboration Order

Generics & Elaboration Order

[Andrew Logue]

Hello,

I need help to instantiate a generic package with an actual parameter whose 
value must be determined at run-time.

Please take a peek at the following pseudo-code:


package body main is

  Actual_Parameter : Boolean := False;
  package New_Pack is new Generic_Package( Parameter => Actual_Parameter );

  begin  -- package elaboration code

    if some_package.embedded_computer_type = some_package.type_A then

      Actual_Parameter := True;
 
    else

      Actual_Parameter := False;

    end if;


end main;

Given the above code, and the embedded computer type is type_A, will 
New_Pack be instantiated with an Actual_Parameter value of True or False?

i.e. does the code in package body execute before the generic is 
instantiated?


Thank you for your feedback,

Andrew Logue,
Computing Devices Canada.

Re: Generics & Elaboration Order

[Robert Dewar]

In article <8F2E930FAandrewloguecdcgycom@142.77.1.194>,
  nospam.andrew.logue@cdcgy.com (Andrew Logue) wrote:
> i.e. does the code in package body execute before the generic
> is instantiated?

No, certainly not

Generally the linear elaboration semantics of Ada mean that
an expression is evaluated when you come to it. There are
very few exceptions to this rule (default expressions in
specs are one case, generic templates are the other).


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Re: Generics & Elaboration Order

[Robert A Duff]

nospam.andrew.logue@cdcgy.com (Andrew Logue) writes:

> package body main is
> 
>   Actual_Parameter : Boolean := False;
>   package New_Pack is new Generic_Package( Parameter => Actual_Parameter );
> 
>   begin  -- package elaboration code
> 
>     if some_package.embedded_computer_type = some_package.type_A then
> 
>       Actual_Parameter := True;
>  
>     else
> 
>       Actual_Parameter := False;
> 
>     end if;
> 
> 
> end main;
> 
> Given the above code, and the embedded computer type is type_A, will 
> New_Pack be instantiated with an Actual_Parameter value of True or False?

I presume Parameter is mode 'in'.  In that case, False is passed.

If Parameter is mode 'in out', then it's like a renaming,
but I don't think that's what you're talking about.

> i.e. does the code in package body execute before the generic is 
> instantiated?

No.

- Bob

Re: Generics & Elaboration Order

[ANTHONY GAIR]

you could try (before package is referred to .)

pragma elaborate(name_of_package);

 this will force the compiler to pull this package in, before compilation of
the rest of the code.
**********************************************

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Re: Generics & Elaboration Order

[Robert A Duff]

anthonygair@aol.comremoveme (ANTHONY GAIR) writes:

> pragma elaborate(name_of_package);

You should almost always use pragma Elaborate_All instead of pragma
Elaborate.

- Bob

Re: Generics & Elaboration Order

[ANTHONY GAIR]

whats the difference Bob ?
**********************************************

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Re: Generics & Elaboration Order

[Robert Dewar]

In article <20000518204425.29388.00000121@ng-cm1.aol.com>,

> whats the difference Bob ?

[between Elaborate and Elaborate_All]

This is a pretty standard issue, any decent Ada text book
should describe how Elaborate_All works. See also the
Ada Rationale for a discussoin of the need for this
new feature.

Don't assume that brief posts to CLA, even if people are
willing to repeat standard stuff, substitute for the careful
and complete discussions that can easily be located in
appropriate reference materials. The Rationale is a particularly
useful document.



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