Hacking "method variables"

Markus Gerwinski markus at gerwinski.de
Tue Jun 13 20:18:01 CEST 2006


Frank Heckenbach wrote:
> Markus Gerwinski wrote:
> > The point is: I'm writing an API where I'd like to give the application
> > developer the possibility to pass a method as an initialization parameter to
> > the constructors of some objects. For example to an intermediate wrapper that
> > turns rows of an SQL result set into objects and vice versa.
> 
> Can you expand a bit on this example? I'm not sure I can really
> imagine why this is really necessary? Are the methods that you pass
> defined in the library and/or the application? Do you need to pass
> arbitrarily many different methods per object type (so virtual
> methods won't do)? Do you need it for static and/or virtual methods?

The methods will be defined in the application. The API will provide a "method
variable" parameter to pass them. I don't quite understand the question with
the many different methods per object. As for static/virtual, I believe in
99% of all cases static methods will do; however, I'm not sure if the case
might arise some day I need to pass a virtual (or even abstract) method.

Imagine something like this:


  (* Code of the API: *)

  type

    tGetIntegerAttr = Function: integer; method;
    tSetIntegerAttr = Procedure ( aValue: integer ); method;

    tDBReader = object
      ...
      Procedure addAttribute (
        const aFieldName: string;
	aGetter: tGetIntegerAttr;
	aSetter: tSetIntegerAttr );
      ...
    end (* tDBReader *);


tDBReader is, in this case, a generic object to read/write database tables and
transform them into (different types of) objects.


  (* Code of the application: *)

  type

    tPerson = object
      ...
      Function getYearOfBirth: integer;
      Procedure setYearOfBirth ( aValue: integer );
    end (* tPerson *);

  var
    PersonReader: pDBReader;

  (* And inside some DB access initialization: *)
    ...
    PersonReader^ .addAttribute (
      'YearOfBirth', tPerson.getYearOfBirth, tPerson.setYearOfBirth );
    ...


Calling addAttribute like this should, in my example, tell the PersonReader
instance to take, for any given tPerson instance, the year of birth out of the
'YearOfBirth' field in the according database table and pass it to the tPerson
using the setYearOfBirth method; when writing it back to the database, the
PersonReader shall use the getYearOfBirth method to get the value out of the
object and write it back into the field 'YearOfBirth' in the DB.

I assume getYearOfBirth and setYearOfBirth will almost always be static
methods. However, as I said above, I cannot say for sure I won't some day have
a situation where I need a virtual or abstract one.





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