Re: Elaboration order

[stt@henning.camb.inmet.com (Tucker Taft)]

DenReimer (denreimer@aol.com) wrote:
: Here's something that I'm trying to do in Ada83 and Ada95:

: with GENERIC_PACKAGE;
: package A_PKG is
:    package INSTANCE is new GENERIC_PACKAGE ( ... );
:    X : INTEGER := INSTANCE.SOME_FUNCTION;
: end A_PKG;

: The elaboration of X requires that the instantiated package INSTANCE be
: elaborated first (both spec and body.)  The compiler will not let me use
: pragma elaborate (INSTANCE);

You should write:
   with Generic_Package;
   pragma Elaborate(Generic_Package);

: It is not clear to me what the elaboration order should be in the case of
: nested packages.  I would like to assume that for nested packages, the
: elaboration is also nested.  In the case of generic instantiations, both
: spec and body would be elaborated at the point of instantiation. 

All true.

: Unfortunately, the LRM does not seem to make any requirements regarding
: elaboration order for nested packages.  

Not true.

: ... I've tried the above source code
: with Ada/ED and an Alsys Ada83 compiler and it seems to work OK.

: When I use gnat, I get a program error.  The Ada95 pragmas used for
: controlling elaboration order seem to only apply to library units.  

That's true in Ada 83 as well.

: ... In the
: example above, I believe that INSTANCE is a program unit but not a library
: unit.  Is there some way I can force the elaboration of INSTANCE to occur
: immediately?  

Your problem is not the elaboration of Instance.  It is the elaboration
of the body of Generic_Package.  You are falling afoul of the
elaboration check that takes place when you instantiate a generic,
which requires that the generic body has already been elaborated.

As a general rule, if a compilation unit has a "with" for a generic, 
it should also have a "pragma Elaborate" for it, if it contains 
any library-level instantiations of the generic.

By the way, you are in good company.  Several of the 2.0.1 ACVC tests
forget this general rule.

: ...
: Dennis Reimer

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA