9.6 Common Elaboration-model Traits

All three GNAT models are able to detect elaboration problems related to dispatching calls and a particular kind of ABE referred to as `guaranteed ABE'.

• * `Dispatching calls'

  GNAT installs run-time checks for each primitive subprogram of each tagged type defined in a partition on the assumption that a dispatching call invoked at elaboration time will execute one of these primitives. As a result, a dispatching call that executes a primitive whose body has not been elaborated yet will raise exception Program_Error at run time. The checks can be suppressed using pragma Suppress (Elaboration_Check).

• * `Guaranteed ABE'

  A guaranteed ABE arises when the body of a target is not elaborated early enough, and causes all scenarios that directly execute the target to fail.

  package body Guaranteed_ABE is
     function ABE return Integer;
  
     Val : constant Integer := ABE;
  
     function ABE return Integer is
     begin
        ...
     end ABE;
  end Guaranteed_ABE;
  

  In the example above, the elaboration of Guaranteed_ABE’s body elaborates the declaration of Val. This invokes function ABE, however the body of ABE has not been elaborated yet. GNAT emits similar diagnostics in all three models:

  1. package body Guaranteed_ABE is
  2.    function ABE return Integer;
  3.
  4.    Val : constant Integer := ABE;
                                  |
     >>> warning: cannot call "ABE" before body seen
     >>> warning: Program_Error will be raised at run time
  
  5.
  6.    function ABE return Integer is
  7.    begin
  8.       ...
  9.    end ABE;
  10. end Guaranteed_ABE;
  

Note that GNAT emits warnings rather than hard errors whenever it encounters an elaboration problem. This is because the elaboration model in effect may be too conservative, or a particular scenario may not be elaborated or executed due to data and control flow. The warnings can be suppressed selectively with pragma Warnigns (Off) or globally with compiler switch -gnatwL.