Chapter 7: Portability - TOC - 7.1 FUNDAMENTALS

7.1.9 Arbitrary Order Dependencies

guideline

Avoid depending on the order in which certain constructs in Ada are evaluated.

example

The output of this program depends upon the order of evaluation of subprogram parameters, but the Ada Reference Manual (1995, §6.4) specifies that these evaluations are done in an arbitrary order:

package Utilities is
   function Unique_ID return Integer;
end Utilities;

package body Utilities is

   ID : Integer := 0;

   function Unique_ID return Integer is
   begin
      ID := ID + 1;
      return ID;
   end Unique_ID;

end Utilities;

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with Ada.Text_IO;
with Utilities; use Utilities;
procedure P is
begin
   Ada.Text_IO.Put_Line (Integer'Image(Unique_ID) & Integer'Image(Unique_ID));
end P;

If the parameters to the "&" function are evaluated in textual order, the output is:

1 2

If the parameters are evaluated in the reverse order, the output is:

2 1

rationale

The Ada language defines certain evaluations to occur in arbitrary order (e.g., subprogram parameters). While a dependency on the order of evaluation may not adversely affect the program on a certain implementation, the code might not execute correctly when it is ported. For example, if two actual parameters of a subprogram call have side effects, the effect of the program could depend on the order of evaluation (Ada Reference Manual 1995, §1.1.4). Avoid arbitrary order dependencies, but also recognize that even an unintentional error of this kind could prohibit portability.