Re: maintenance of overriding subprograms

["John G. Volan" <johnv@ac3i.dseg.ti.com>]

Tom Moran wrote:
> 
> A related error, not, unfortunately, solved by that syntax occurs when a
> maintainer erroneously believes some parameter to have a default, and
> accordingly removes or comments it out in a call, eg
>   package Basic is
>     type Object_Type is tagged ...
>     procedure Create_Object(X : Object_Type;
>         Name : in String; Size, Position : in Rectangles);
> ...
>   package Fancy is
>     type Fancy_Object_Type is new Basic.Object_Type with ...
>     procedure Create_Object(X : Fancy_Object_Type;
>         Name : in String; Size, Position : in Rectangles;
>         Title : in String);
> ...
> A call
>   Create_Object(X -- X is a Fancy_Object
>      "It's name", The_Size, The_Position, "");
> is erroneously changed to
>   Create_Object(X -- X is a Fancy_Object
>      "It's name", The_Size, The_Position);
> under the misapprehension that Title defaults to the null string.  But
> in fact this has changed from a call on Fancy.Create to an inherited
> call on Basic.Create  Manual proofreading of the code is fairly likely
> to miss the error, but what's especially insidious is that the behavior
> is probably quite similar, so the fact there is a problem may not show
> up for some time.

    package Basic is
    
You can avoid this sort of thing by adopting the following practices:

(1)  Give controlling subprogram parameters names that reflect their
level of abstraction. Don't use an "inherited" identifier.  e.g.:

  package Basic is
    type Basic_Object_Type is tagged ...
    procedure Create
      (Basic_Object    : in out Basic_Object_Type; 
        Name           : in     String;
        Size, Position : in     Rectangle_Type);
...
  package Fancy is
    type Fancy_Object_Type is new Basic.Basic_Object_Type with ...
    procedure Create
      (Fancy_Object   : in out Fancy_Object_Type; 
       Name           : in     String;
       Size, Position : in     Rectangle_Type;
       Title          : in     String);
...

(2) Use named parameter association syntax religiously, e.g.:

  Fancy.Create 
    (Fancy_Object => Your_Fancy_Object -- _obviously_ a Fancy_Object
     Name         => "It's name", 
     Size         => The_Size,
     Position     => The_Position,
     Title        => "");

If you then try to change it to:

  Fancy.Create 
    (Fancy_Object => Your_Fancy_Object -- _obviously_ a Fancy_Object
     Name         => "It's name", 
     Size         => The_Size,
     Position     => The_Position);

then you get a compilation error.

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