Re: AQ&S Guidance on pragma Elaborate_Body

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Responding To Bob Duff's example

   generic
   package G1 is ... end G1;

   package body G1 is ... end G1;

   generic
   package G2 is ... end G2;

   with G1;
   package body G2 is
       package Inst_1 is new G1;
   end G2;

   with G2; pragma Elaborate(G2);
   package P is
       package Inst_2 is new G2;
   end P;

Matthew Heaney asks

  But isn't G2 supposed to elaborate G1?

  with G1;
  pragma Elaborate (G1);   <<<---
  package body G2 is
      package Inst_1 is new G1;
  end G2;

  Isn't G2 supposed to take care of it's own business, and make sure G1 is
  elaborated?  This is analogous to P making sure G2 is elaborated.

Robert Dewar replies

In general the answer is no. G2 has no elaboration problem (generic packages
themselves require nothing to be elaborated). It is the instance of G2
that might need elaboration.

If no one ever instantiates G2 in elaboration code, then there is absolutely
no need to elaborate the body of G1 before the spec of G2. This requirement
comes from the fact that P instantiates G2 at elaboration time. Thus P
must take responsibility for the elaboration, and do a pragma Elaborate_All,
rather than a pragma Elaborate.

Note: This is exactly the kind of automatic Elaborate_All that the new
version of GNAT does by default. It will also on request output warnings
for missing Elaborate_All pragmas.